I found the following :
" The purpose of a tower-top amplifier, or TTA, is to improve receiver
sensitivity at the repeater site. Good sensitivity at the repeater is
especially important in land mobile radio systems because it overcomes
some of the link imbalance created by the high-power repeater
transmitter. "
But, How is the above possible ? Any links / documents pls !
"karthikbg" <karthik.balaguru@lntinfotech.com> wrote in message
news:1167823253.849868.291060@42g2000cwt.googlegro ups.com...
| Hi,
|
| I found the following :
| " The purpose of a tower-top amplifier, or TTA, is to improve receiver
| sensitivity at the repeater site. Good sensitivity at the repeater is
| especially important in land mobile radio systems because it overcomes
| some of the link imbalance created by the high-power repeater
| transmitter. "
|
| But, How is the above possible ? Any links / documents pls !
It's possible (worked with such systems for years) but be aware that with
few exceptions the performance improvements are minimal. Further plan on
having replacement hardware in hand and to do some high work as it takes
only a light lighting strike (some cases not direct but near by) to kill the
entire system.
Recall the TV commercial "can you hear me now?" and the old ham adage 'you
can't work'um if you can't hear 'um'
FIWI I go for T/R up when I have easy/safe access to the hardware but avoid
the option whenever I can if there is any problem accessing the hardware.
this to the point of refusing the deal.
karthikbg wrote:
> Hi,
>
> I found the following :
> " The purpose of a tower-top amplifier, or TTA, is to improve receiver
> sensitivity at the repeater site. Good sensitivity at the repeater is
> especially important in land mobile radio systems because it overcomes
> some of the link imbalance created by the high-power repeater
> transmitter. "
>
> But, How is the above possible ? Any links / documents pls !
>
> Thx in advans,
> Karthik Balaguru
>
Hi,
Which part you don't understand? High power TX needs matching RX
sensitivity to maintain balance and amp. is always installed at the
mast not at the bottom of tower(this defeats the purpose).
On 3 Jan 2007 03:20:53 -0800, "karthikbg"
<karthik.balaguru@lntinfotech.com> wrote:
>I found the following :
>" The purpose of a tower-top amplifier, or TTA, is to improve receiver
>sensitivity at the repeater site. Good sensitivity at the repeater is
>especially important in land mobile radio systems because it overcomes
>some of the link imbalance created by the high-power repeater
>transmitter. "
>
>But, How is the above possible ? Any links / documents pls !
Land mobile (VHF/UHF) is somewhat different than 2.4GHz. The big
difference is coax cable losses. At 2.4GHz, they're MUCH higher. If
you have a tower, and about 100ft of reasonable coax cable (LMR-400),
you'll see about 8dB of coax loss at 2.4Ghz. Put an antenna on top of
the tower, add this 100ft of coax, and plant the access point in the
shelter building. You'll get terrible reception because the 7dB of
loss will reduce the range to about 40% of what it might be if the
access point were located at the antenna (on top of the tower)[1]. In
some cases, it's impractical to put the access point on top of the
tower. For example, visualize climbing the tower in a storm. So,
instead of the whole access point, a bi-directional (switched)
amplifier is installed on the top of the tower. This eliminates the
coax cable loss in both directions, and dramatically improves the
range.
Unfortunately, the link is a bit muddled. While a tower top amplifier
can usually improve range in both directions (xmit and receive), it
more often creates the imbalance that the article suggests. Some
tower top 2.4GHz amplifiers transmit at 1 watt, while the client radio
might be lucky and transmit at perhaps 0.035 watts. That's a major
imbalance caused by the TTA.
The article you found (and didn't bother citing the source) is about
land mobile radio. In land mobile, repeaters typically have power
outputs of perhaps 40 to 100 watts depending on system requirements.
The typical handheld will deliver between 1 and 5 watts. That's a
rather large imbalance and problems with handhelds hearing the
repeater, but not being able to respond.
The TTA that I'll guess your article is discussing is not a
bi-directional amplifier, but rather a receiver multicoupler and
amplifier system, where a single antenna and tower top receive
amplifier is used to feed a building full of VHF or UHF receivers.
Sometimes, the amplifier is cyrogenically cooled to obtain the best
possible sensitivity. By dramatically improving the receiver
sensitivity, some of the imbalance in transmit powers can be
compensated.
"Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
news:7lfop2965du7cfr2bj2llc0u932vrv6l4c@4ax.com...
> On 3 Jan 2007 03:20:53 -0800, "karthikbg"
> <karthik.balaguru@lntinfotech.com> wrote:
>
> >I found the following :
> >" The purpose of a tower-top amplifier, or TTA, is to improve receiver
> >sensitivity at the repeater site. Good sensitivity at the repeater is
> >especially important in land mobile radio systems because it overcomes
> >some of the link imbalance created by the high-power repeater
> >transmitter. "
> >
> >But, How is the above possible ? Any links / documents pls !
>
> Land mobile (VHF/UHF) is somewhat different than 2.4GHz. The big
> difference is coax cable losses. At 2.4GHz, they're MUCH higher. If
> you have a tower, and about 100ft of reasonable coax cable (LMR-400),
> you'll see about 8dB of coax loss at 2.4Ghz. Put an antenna on top of
> the tower, add this 100ft of coax, and plant the access point in the
> shelter building. You'll get terrible reception because the 7dB of
> loss will reduce the range to about 40% of what it might be if the
> access point were located at the antenna (on top of the tower)[1]. In
> some cases, it's impractical to put the access point on top of the
> tower. For example, visualize climbing the tower in a storm. So,
> instead of the whole access point, a bi-directional (switched)
> amplifier is installed on the top of the tower. This eliminates the
> coax cable loss in both directions, and dramatically improves the
> range.
>
> Unfortunately, the link is a bit muddled. While a tower top amplifier
> can usually improve range in both directions (xmit and receive), it
> more often creates the imbalance that the article suggests. Some
> tower top 2.4GHz amplifiers transmit at 1 watt, while the client radio
> might be lucky and transmit at perhaps 0.035 watts. That's a major
> imbalance caused by the TTA.
>
> The article you found (and didn't bother citing the source) is about
> land mobile radio. In land mobile, repeaters typically have power
> outputs of perhaps 40 to 100 watts depending on system requirements.
> The typical handheld will deliver between 1 and 5 watts. That's a
> rather large imbalance and problems with handhelds hearing the
> repeater, but not being able to respond.
>
> The TTA that I'll guess your article is discussing is not a
> bi-directional amplifier, but rather a receiver multicoupler and
> amplifier system, where a single antenna and tower top receive
> amplifier is used to feed a building full of VHF or UHF receivers.
> Sometimes, the amplifier is cyrogenically cooled to obtain the best
> possible sensitivity. By dramatically improving the receiver
> sensitivity, some of the imbalance in transmit powers can be
> compensated.
>
> [1] 6dB loss is half the range
> 12dB loss is 1/4 the range
When working in the land mobile field, and cellular field.
We would use TTA at the top of the antenna for the base station recieve.
This allowed us to recieve the weaker power mobile units. Using the TTA on
our receive side only, allowed us to balance our high base station transmit
with the low power mobiles.
We would also use Bi directional amps to bring the signal into tunnels or
buildings.
We would either use leaky cable or a system of antennas inside the building
or tunnel.
> --
> # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
> # 831-336-2558 jeffl@comix.santa-cruz.ca.us
> # http://802.11junk.comjeffl@cruzio.com
> # http://www.LearnByDestroying.com AE6KS
karthikbg wrote:
> I found the following :
> " The purpose of a tower-top amplifier, or TTA, is to improve receiver
> sensitivity at the repeater site. Good sensitivity at the repeater is
> especially important in land mobile radio systems because it overcomes
> some of the link imbalance created by the high-power repeater
> transmitter. "
Generally speaking, you'll find several references from system
manufactures explaining that receiver amplifiers introduce additional
noise in the system and the overall improvement is negligible for WiFi.
"decaturtxcowboy" <nope_none_@nowayspam.com> wrote in message
news:3%5nh.13520$yC5.145@newssvr27.news.prodigy.ne t...
> karthikbg wrote:
> > I found the following :
> > " The purpose of a tower-top amplifier, or TTA, is to improve receiver
> > sensitivity at the repeater site. Good sensitivity at the repeater is
> > especially important in land mobile radio systems because it overcomes
> > some of the link imbalance created by the high-power repeater
> > transmitter. "
>
> Generally speaking, you'll find several references from system
> manufactures explaining that receiver amplifiers introduce additional
> noise in the system
Which is why you mount them at the top of the tower near the antenna.
If you mount them at the bottom of the tower, than any gain is negated by
the noise from the transmission line being amplified by the TTA.
Dana wrote:
> "decaturtxcowboy" <nope_none_@nowayspam.com> wrote in message
>> Generally speaking, you'll find several references from system
>> manufactures explaining that receiver amplifiers introduce additional
>> noise in the system
>
> Which is why you mount them at the top of the tower near the antenna.
> If you mount them at the bottom of the tower, than any gain is negated by
> the noise from the transmission line being amplified by the TTA.
The coax doesn't produce any noise at all...period. Its the noise
introduced by the receiver amplifier. I seem to recall TerraWave said the
noise introduced by a receiver amplifier negates any system gain and they
do not even add the additional gain into the overall gain calculations.
decaturtxcowboy wrote:
> Dana wrote:
>
>> "decaturtxcowboy" <nope_none_@nowayspam.com> wrote in message
>>
>>> Generally speaking, you'll find several references from system
>>> manufactures explaining that receiver amplifiers introduce additional
>>> noise in the system
>>
>>
>> Which is why you mount them at the top of the tower near the antenna.
>> If you mount them at the bottom of the tower, than any gain is negated by
>> the noise from the transmission line being amplified by the TTA.
>
>
> The coax doesn't produce any noise at all...period. Its the noise
> introduced by the receiver amplifier. I seem to recall TerraWave said
> the noise introduced by a receiver amplifier negates any system gain and
> they do not even add the additional gain into the overall gain
> calculations.
Hi,
There is no signal in this world without noise component. If signal and
noise are amplified together and still sginal is above noise, that is a
over all gain. Ever heard of knee of quieting or S/N ratio? Some devices
are less noisier than others in amplifier application. Some LNAs can be
even kept cool to lower the noise figure(or noise floor).
On Jan 5, 3:19 am, decaturtxcowboy <nope_no...@nowayspam.com> wrote:
> The coax doesn't produce any noise at all...period. Its the noise
> introduced by the receiver amplifier. I seem to recall TerraWave said the
> noise introduced by a receiver amplifier negates any system gain and they
> do not even add the additional gain into the overall gain calculations.
You haven't seen the crappy coax cable to connector interfaces that
I've had to deal with. They not only produce noise, but they also act
as diode mixers to produce intermod. While the coax itself doesn't
produce any added noise, any dissimilar metals in the system (aluminum
coax outer conductor to nickel plated connector) will produce noise and
intermod.
Anyway, mounting the receive amplifier at the antenna eliminates the
reduction in sensitivity cause by the coax loss. If I assume that the
noise figure of the receiver input amplifier and the tower top
amplifier are reasonably low (they usually are), and the gain of the
tower top amplifier is a few dB more than the coax loss, what little
added noise the tower top amplifier contributes will not have a
significant effect on overall sensitivity and system noise figure
(either better or worse). Calculations if you really want them (say
no, I'm busy).
decaturtxcowboy wrote:
> Tony Hwang wrote:
>
>> ome LNAs can be
>> even kept cool to lower the noise figure(or noise floor).
>
>
> For a price that makes them not cost effective.
Hi,
If you work on radio astronomy for an example!
"decaturtxcowboy" <nope_none_@nowayspam.com> wrote in message
news:7vqnh.57219$qO4.14606@newssvr13.news.prodigy. net...
> Dana wrote:
> > "decaturtxcowboy" <nope_none_@nowayspam.com> wrote in message
> >> Generally speaking, you'll find several references from system
> >> manufactures explaining that receiver amplifiers introduce additional
> >> noise in the system
> >
> > Which is why you mount them at the top of the tower near the antenna.
> > If you mount them at the bottom of the tower, than any gain is negated
by
> > the noise from the transmission line being amplified by the TTA.
>
> The coax doesn't produce any noise at all...period.
Everything produces noise.
And in the applications I was describing we had anywhere from 100 to 600
feet of cable to deal with.
>Tony Hwang wrote:
>> ome LNAs can be
>> even kept cool to lower the noise figure(or noise floor).
>For a price that makes them not cost effective.
True. That's because most of the technology in use is commercialized
military technology and is also done on a bulk scale. My crystal ball
sees that changing with the introduction of SDF radios, where most of
the receiver is cryo cooled using SME cooling. It's kinda a marginal
proposition to cool just the LNA. However, cooling the LNA, filters,
and A/D converter in the front end of an SDF radio makes lots of sense
and may be the only way to get the noise induced digitization errors
down to a reasonable level for SDF radios past about 18bits.
Real-soon-now.
There are also cryogenic ceramic cavity filters and duplexers that
offer very low loss and high Q filters. I've played with a rx
amplifier multi-coupler for a shared receive site, where it makes
sense because the cost can be distributed among perhaps 8-16 radios.
What's really expensive for cell sites is the cost of the electricity
to run the current coolers. To get an LNA or ceramic cavity filter
down to about 50-150K necessary to use room temperature ceramics, it
takes about 60-80 watts of power. Multiply that by a large number of
cell sites, and the cost of electricity alone will kill the idea[2].
I couldn't find a good overview of the current trends in SME. This
covers most of the stuff:
<http://www.cryogenicsociety.org/cryo_central/telecommunications.php>
<http://www.suptech.com/articles.htm>
[1] Buzzwords:
SDF = Software Defined Radio
SME = Superconducting MicroElectronics
LNA = Low noise amplifier
A/D = Analog to Digital
[2] Bulk max rate commercial electricity is about $0.35/kw-hr. At 80
watts, that's about $250/year for just the cooler power.
On Jan 6, 10:12 am, "Dana" <raff...@yahoo.com> wrote:
> Everything produces noise.
> And in the applications I was describing we had anywhere from 100 to 600
> feet of cable to deal with.
Wrong. Coax cable does not normally generate any noise by itself.
There are some conditions where the coax cable can be convinced to
create some noise, such a electrolytic action or braid rubbing against
itself. Some microwave test set manufacturers sell low noise coax test
cable to minimize these effects. However, a properly installed
conventional coax run, no matter how long, does not generate any noise.
At 2.4GHz 100ft of LMR-600 is 4.4dB/100ft. You'll loose about 60% of
your power in the coax. 100ft is actually not too horrible unless you
have to squeeze every bit of performance out of the system. However,
at much more than that, you'll need to do something else.
Please note that bi-directional 2.4Ghz amplifiers without AGC require a
very specific length of coax cable and AP output power to insure that
the tx power amplifier is properly driven and remains linear. Those
with AGC are bit more forgiving. I've had miserable experiences with
2.4GHz tower top amplifiers. Details on request. At 600ft, I would
look into a PoE system and forget both the coax cable and TTA.
"Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
news:1168110999.955734.285650@42g2000cwt.googlegro ups.com...
>
>
> On Jan 6, 10:12 am, "Dana" <raff...@yahoo.com> wrote:
> > Everything produces noise.
> > And in the applications I was describing we had anywhere from 100 to 600
> > feet of cable to deal with.
>
> Wrong. Coax cable does not normally generate any noise by itself.
While I agree with you on most everything you post.
Every thing produces what is called thermal noise.
> There are some conditions where the coax cable can be convinced to
> create some noise, such a electrolytic action or braid rubbing against
> itself. Some microwave test set manufacturers sell low noise coax test
> cable to minimize these effects. However, a properly installed
> conventional coax run, no matter how long, does not generate any noise.
>
> At 2.4GHz 100ft of LMR-600 is 4.4dB/100ft. You'll loose about 60% of
> your power in the coax. 100ft is actually not too horrible unless you
> have to squeeze every bit of performance out of the system. However,
> at much more than that, you'll need to do something else.
>
> Please note that bi-directional 2.4Ghz amplifiers without AGC require a
> very specific length of coax cable and AP output power to insure that
> the tx power amplifier is properly driven and remains linear. Those
> with AGC are bit more forgiving. I've had miserable experiences with
> 2.4GHz tower top amplifiers. Details on request. At 600ft, I would
> look into a PoE system and forget both the coax cable and TTA.
And as an aside, those 600 foot cable runs were in the land mobile trunked
and conventional radio systems, in the 800 and 900Mhz band.
"Dana" <raff242@yahoo.com> wrote in message
news:12pvuvt2ov2o4c4@corp.supernews.com...
>
> "Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
> news:1168110999.955734.285650@42g2000cwt.googlegro ups.com...
> >
> >
> > On Jan 6, 10:12 am, "Dana" <raff...@yahoo.com> wrote:
> > > Everything produces noise.
> > > And in the applications I was describing we had anywhere from 100 to
600
> > > feet of cable to deal with.
> >
> > Wrong. Coax cable does not normally generate any noise by itself.
Here is an article that describes noise in RF systems http://telephonyonline.com/wireless/...luating_noise/
All matter at temperatures above absolute zero (0K, about -460F) radiates
electromagnetic energy. The amount of energy is related to temperature --
the hotter the matter, the more energy is radiated. This energy is described
by Boltzmann's Constant, 'k' (k = -198.6dBm/degreesK-Hz). This constant,
multiplied by the temperature of the matter a receiver views and the system
bandwidth, yields an irreducible background noise against which a desired
signal must compete. This is thermal noise.
In a cellular system, the receiving antennas are designed to view the ground
around the site because that's where the subscribers are. The ground
temperature varies, but at 80F, it's about 300K (T=300K). RF engineers
typically use this number as a rule of thumb.
The receiver bandwidth varies depending on the technology, but the same
principles hold for all technologies. EAMPS, for example, uses 30kHz-wide
channels. Receiver bandwidth is a bit less than 30kHz, for rejection of
adjacent channels. Assume the typical EAMPS receiver has a bandwidth of
25kHz (B=25,000Hz). By making this assumption, you can calculate the amount
of noise an EAMPS receiver will have in its passband if it contributes no
noise of its own.
This receiver thermal noise floor often is referred to as 'kTB.' In the
example, assume consistent units:
kTB = -198.6 + 10 Log(300)
+ 10 Log(25,000) in dBm
kTB = -129.8dBm
Thus, if you build a perfect EAMPS cellular receiver, it would have -129.
8dBm of noise in its passband competing with the wanted signal.
CABLE LOSS Cable, filters and other passive elements exhibit a loss and
produce thermal noise.
If a cable (or other lossy element) has 10dB of loss, it will attenuate the
desired signal as well as the input noise by 10dB. But at the output of the
cable, you will see noise at least equal to kTB because the cable itself
contributes it.
If you put a signal into the cable at -100dBm over a thermal noise of -129.
8dBm, you have a signal-to-noise ratio of 29.8dB at the cable input. At the
cable output, the signal has been attenuated by 10dB to -110dBm. The noise
you put into the cable also has dropped the same amount, to -139.8dBm. But
the cable contributes its noise floor of -129.8dBm, so the combined
(uncorrelated) noise terms are -129.8dBm. The resulting signal-to-noise
ratio is only 19.8dB at the cable output. You sacrifice 10dB of
signal-to-noise ratio. This is why you spend money on 7/8", 15/8" or larger
coax at cell sites to reduce this loss.
>Jeff Liebermann wrote:
>> You haven't seen the crappy coax cable to connector interfaces that
>> I've had to deal with.
>Shopping at Radio Shack still?? [just kidding]
Yech. They sell some of the worst coax cable I've ever seen. I
bought some RG-58a/u clone once that I swear had less than 75%
coverage on the braid. (Don't mention PL-259 connectors). The
unplated copper in the coax also seemed to corrode more easily than
better cable. The air gap in the loose braid was perfect for
capillary action sucking water into the coax. Strangely, their
satellite grade RG-6/u cables seem just fine (except for the cheezy F
connector).
Nickel plated connectors are know to cause intermod. There was an
article in MRT magazine on the topic:
"Intermod and Connectors: Silver Plate Beats Nickel"
by Manny Gutsche, Mobile Radio Technology, Mar. 1992.
It too old to be on their web pile.
As a result of the above article, several site owners went on a
rampage to remove nickel plated connectors. One site, that I was
involved with at the time, showed a rather dramatic improvement in
intermod reduction.
Anyway, most Radio Shack connectors and adapters are nickel plated
except the gold plated variety.
Dana wrote:
> "Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
> news:1168110999.955734.285650@42g2000cwt.googlegro ups.com...
>>
>> On Jan 6, 10:12 am, "Dana" <raff...@yahoo.com> wrote:
>>> Everything produces noise.
>>> And in the applications I was describing we had anywhere from 100 to 600
>>> feet of cable to deal with.
>> Wrong. Coax cable does not normally generate any noise by itself.
>
> While I agree with you on most everything you post.
> Every thing produces what is called thermal noise.
For all intent and purposes...the thermal noise generated by a passive coax
line in a typical LMR and WiFi system is NOT EVEN a considered issue.
The original OP was referring to LMR and thinking of WiFi...not a million
dollar research facility working with signal levels so low that can't be
observed with a diving bell.
Ergo, it still stands - coax does not normally generate any noise by itself
(miscellaneous metal junction intermod excluded) that needs to be entered
in any LMR, WiFi, or WiMax deployment calculations.
"decaturtxcowboy" <nope_none_@nowayspam.com> wrote in message
news:0GYnh.57474$qO4.21066@newssvr13.news.prodigy. net...
> Dana wrote:
> > "Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
> > news:1168110999.955734.285650@42g2000cwt.googlegro ups.com...
> >>
> >> On Jan 6, 10:12 am, "Dana" <raff...@yahoo.com> wrote:
> >>> Everything produces noise.
> >>> And in the applications I was describing we had anywhere from 100 to
600
> >>> feet of cable to deal with.
> >> Wrong. Coax cable does not normally generate any noise by itself.
> >
> > While I agree with you on most everything you post.
> > Every thing produces what is called thermal noise.
>
> For all intent and purposes...the thermal noise generated by a passive
coax
> line in a typical LMR and WiFi system is NOT EVEN a considered issue.
Actually in system design it is considered
>
> The original OP was referring to LMR and thinking of WiFi...not a million
> dollar research facility working with signal levels so low that can't be
> observed with a diving bell.
LMR and Cellular both have to deal with noise floor issues, and all
contributions of noise, including cables and coax.
>
> Ergo, it still stands - coax does not normally generate any noise by
itself
At 54Mbits/sec, the typical receiver sensitivity is -68dBm. Actually,
it seems to vary from -66dBm to -74dBm). However, the -68dBm is at a
BER of 10^-5. OFDM at 54Mbits/sec requires a minimum SNR of 24.6dB.
Therefore, the real noise floor for the receiver front end is:
-68dBm -24.6dBm = -92.6dBm
For calculations at other speeds, I created this table. The typical
receiver is a DLink DI-624.
In a perfect world, the noise floor would be exactly the same value
for all speeds. However, perfection is almost impossible and
variations in demodulator efficiency, digital noise pickup, and test
measurement accuracy result in a rather wide (10dB) range. I just
hate reality.
So, back to the coax cable noise. I'll use the 54Mbit/sec numbers.
With a thermal noise floor of -100dBm, and a receiver noise floor of
-92.6dBm, there is a 7.4dB margin. Since the coax cable contributes
the same amount of noise whether it is a few inches or a few miles
long, the maximum attenuation that can be tolerated in the coax cable
is 7.4dB. Any less attenuation and the coax noise has no effect on
the sensitivity. Any more attenuation, and the coax noise exceeds the
signal level. By that time, the receiver losses from the coax
attenuation will have a larger effect than the added coax noise, so
again the coax noise is negligible. Incidentally, there are a few
assumptions in this simplification that I'm too lazy to detail. The
7.4dB is not the exact number but close enough.
Incidentally, 7.4dB is about 100ft of LMR-400 plus connector losses.
"decaturtxcowboy" <nope_none_@nowayspam.com> wrote in message
news:pM%nh.2899$ji1.1491@newssvr12.news.prodigy.ne t...
> Dana wrote:
> > LMR and Cellular both have to deal with noise floor issues, and all
> > contributions of noise, including cables and coax.
>
> Noise floor issues yes. Coax generated noise, no.
The actual answer is yes, if you want a reliable link
> I don't know what he was
> smoking when he wrote that article.
It is obvious what you know and do not know.
Seems you have not been involved in long distance radio links in the VHF,
UHF, or Microwave bands.
Then you would appreciate what the article was about.
>
> >> Ergo, it still stands - coax does not normally generate any noise by
> >> itself
Like most everything, it sure does generate noise.
But I do not think you understand what noise is being generated and how it
affects receiver sensitivity.
>Dana wrote:
>> LMR and Cellular both have to deal with noise floor issues, and all
>> contributions of noise, including cables and coax.
>
>Noise floor issues yes. Coax generated noise, no. I don't know what he was
>smoking when he wrote that article.
You're both right. Anything that's not at absolute zero will generate
thermal noise. However, the coax cable does NOT generate additional
noise that would be additive. For example, if the front end noise
temperature were 290K, attaching a 50 ohm dummy load to the front end
would not increase the noise level. Putting a piece of coax cable
between the front end and the dummy load would also not increase the
noise level. Replacing the 50 ohm load with an antenna but burying
the antenna inside an RF absorbent box, would also not generate any
additional noise. That's because off of these devices (load, coax,
antenna) are at the same temperature. The equivalent noise
temperature looks exactly like a perfect termination with a 290K noise
source in parallel in all cases.
If the cable generated additional noise, that noise level would need
to be added to the front end thermal noise contribution. This is not
the case and the original article stated the same thing.
A common misconception is that larger objects "retain" more heat and
therefore generate more noise. Nope. It's only the absolute
temperature that determines the noise level. A tiny 50 ohm chip
resistor termination generates exactly the same amount of thermal
noise as a much larger dummy load, antenna, or combination of coax and
antenna.
However, when we attach an antenna to the system, and point it at
various thermal objects, things change. For example, a 0dBi antenna
pointed at the ground will have exactly the same thermal noise
contribution as the original 290K and will not increase the noise
level. The same antenna pointed at the sun will have a 25,000K
contribution and will probably bury the signal bandwidth in added
noise. Pointing it at empty space will at 3K (cosmic background
radiation).
There are even microwave fire detectors, that use the microwave
emissions of a fire to detect and find fires.
<http://nts.uni-duisburg.de/publications/paper/AUBE04_Willms_et_al.pdf>
<http://www.fire.nist.gov/bfrlpubs/fire01/PDF/f01041.pdf>
The nice part is that microwave frequencies go through may objects
allowing the detector to find hidden hot spots.
Where this become important is when the front end noise figure of the
receiver starts to approach the thermal noise floor. GaAs FET front
ends are close. If cryogenically cooled, even closer. Now, it
becomes important to consider the thermal noise contributions of the
surroundings. However, it's the thermal noise component of whatever
the antenna is looking at that's important, not the coax cable. The
coax, connectors, and antenna only contribute the usual 290K which is
already part of the noise bandwidth equation.
> Dana wrote:
>
>> LMR and Cellular both have to deal with noise floor issues, and all
>> contributions of noise, including cables and coax.
>
>
> Noise floor issues yes. Coax generated noise, no. I don't know what he
> was smoking when he wrote that article.
>
>>> Ergo, it still stands - coax does not normally generate any noise by
>>> itself in the *typical* LMR, cellular, WiMax, or WiFi deployment.
>
>
>
Hi,
What is the definition of noise? Like weed in our garden? Unwated signal
is considered noise. Unwanted plant is weed. Electrically unlinear joint
like coax connector or any joint will generate harmonics.
Temperature which is caused by excited electrons will generate
thermionic noise; shot noise. In my working days I used to work on weak
signal in the range of -80 to 100db and like RX front and gain of 9db
wasa BIG boost making the signal more reliable.
Also RX antenna up high on the tower can induce static discharge, corona
arc, etc. That's why we DC ground antenna.
> Dana wrote:
>
>> LMR and Cellular both have to deal with noise floor issues, and all
>> contributions of noise, including cables and coax.
>
>
> Noise floor issues yes. Coax generated noise, no. I don't know what he
> was smoking when he wrote that article.
Never saw a coax radiating or picking up signal like crazy in real
world? Want to experiment? Raise the SWR on the transmission line
and scan(sniff)the coax along, what do you see?
>
>>> Ergo, it still stands - coax does not normally generate any noise by
>>> itself in the *typical* LMR, cellular, WiMax, or WiFi deployment.
>
>
>
>Never saw a coax radiating or picking up signal like crazy in real
>world? Want to experiment? Raise the SWR on the transmission line
>and scan(sniff)the coax along, what do you see?
Wrong. I do that experiment as a demonstration of common electronic
assumptions (along with the loss through a mess of adapters, water in
the coax, and others). Somewhere along the line, someone mumbled that
VSWR causes RF to radiate down the outside of the coax shield. It
made sense because every time I ran coax to a high VSWR antenna, I had
the coax radiate enough RF to light up fluorescent bulbs in the shack.
However, that's not the way it works. The previous example is an
uncontrolled environment. What's happening is that the coax cable has
become part of the antenna system and will radiate as described. But,
what if I replace the high VSWR antenna with a high VSWR dummy load?
If the reflected signal comes down the outside of the coax, as is
commonly suggested, then it should radiate as badly as the high VSWR
antenna. It doesn't.
You can try your RF sniffer experiment the same way. Put a high VSWR
dummy load on the end of a piece of coax and go sniffing for RF. You
won't find any. As long as the field between the center conductor and
the shield is totally enclosed, you can have a high VSWR termination,
but no radiation outside the shield.
"Jeff Liebermann" <jeffl@comix.santa-cruz.ca.us> wrote in message
news:f8v2q25qtibnk8r915f7o2vi9thoejt5v1@4ax.com...
> Tony Hwang <dragon40@shaw.ca> hath wroth:
>
> >Never saw a coax radiating or picking up signal like crazy in real
> >world? Want to experiment? Raise the SWR on the transmission line
> >and scan(sniff)the coax along, what do you see?
>
> Wrong. I do that experiment as a demonstration of common electronic
> assumptions (along with the loss through a mess of adapters, water in
> the coax, and others).
Water in the coax or feedline, how I hate that, especially here in Alaska
where the water then freezes inside the cable.
Talk about a gremlin type of problem.
Somewhere along the line, someone mumbled that
> VSWR causes RF to radiate down the outside of the coax shield. It
> made sense because every time I ran coax to a high VSWR antenna, I had
> the coax radiate enough RF to light up fluorescent bulbs in the shack.
>
> However, that's not the way it works. The previous example is an
> uncontrolled environment. What's happening is that the coax cable has
> become part of the antenna system and will radiate as described. But,
> what if I replace the high VSWR antenna with a high VSWR dummy load?
> If the reflected signal comes down the outside of the coax, as is
> commonly suggested, then it should radiate as badly as the high VSWR
> antenna. It doesn't.
>
> You can try your RF sniffer experiment the same way. Put a high VSWR
> dummy load on the end of a piece of coax and go sniffing for RF. You
> won't find any. As long as the field between the center conductor and
> the shield is totally enclosed, you can have a high VSWR termination,
> but no radiation outside the shield.
>
>
> --
> Jeff Liebermann jeffl@comix.santa-cruz.ca.us
> 150 Felker St #D http://www.LearnByDestroying.com
> Santa Cruz CA 95060 http://802.11junk.com
> Skype: JeffLiebermann AE6KS 831-336-2558
Jeff Liebermann wrote:
> Tony Hwang <dragon40@shaw.ca> hath wroth:
>
>
>>Never saw a coax radiating or picking up signal like crazy in real
>>world? Want to experiment? Raise the SWR on the transmission line
>>and scan(sniff)the coax along, what do you see?
>
>
> Wrong. I do that experiment as a demonstration of common electronic
> assumptions (along with the loss through a mess of adapters, water in
> the coax, and others). Somewhere along the line, someone mumbled that
> VSWR causes RF to radiate down the outside of the coax shield. It
> made sense because every time I ran coax to a high VSWR antenna, I had
> the coax radiate enough RF to light up fluorescent bulbs in the shack.
>
> However, that's not the way it works. The previous example is an
> uncontrolled environment. What's happening is that the coax cable has
> become part of the antenna system and will radiate as described. But,
> what if I replace the high VSWR antenna with a high VSWR dummy load?
> If the reflected signal comes down the outside of the coax, as is
> commonly suggested, then it should radiate as badly as the high VSWR
> antenna. It doesn't.
>
> You can try your RF sniffer experiment the same way. Put a high VSWR
> dummy load on the end of a piece of coax and go sniffing for RF. You
> won't find any. As long as the field between the center conductor and
> the shield is totally enclosed, you can have a high VSWR termination,
> but no radiation outside the shield.
>
>
Hmmm,
How come a guy miles away picked up my signal when I was folling around
with matched dummy load? When Z is smatched SWR is low but perfect SWR
does not gurantee perfect match. You explained it just now.
Yes, coax can be leaky electrically. Hmmm,
No wonder mil-spec stuffs have double triple shielding compared to
commercial counter part.
purpose of a tower-top amplifier 
Linear Mode
