purpose of a tower-top amplifier

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

--

"karthikbg" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) 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"
<(E-Mail Removed)> 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
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS

"Jeff Liebermann" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On 3 Jan 2007 03:20:53 -0800, "karthikbg"
> <(E-Mail Removed)> 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 (E-Mail Removed)
> # http://802.11junk.com (E-Mail Removed)
> # 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$(E-Mail Removed) 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).