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VK2TV > TECH 28.06.07 11:40l 89 Lines 4387 Bytes #999 (0) @ WW
BID : 23572_VK2TV
Read: GUEST OE7FMI
Subj: Re:APRS and Packet-one aerial
Path: DB0FHN<DB0MRW<DK0WUE<7M3TJZ<ON0AR<IW2OAZ<SR1BSZ<WA7V<KD4YAL<VK2TV
Sent: 070628/1031Z @:VK2TV.#MNC.NSW.AUS.OC #:14214 [Kempsey, QF68JX] $:23572_VK
From: VK2TV@VK2TV.#MNC.NSW.AUS.OC
To : TECH@WW
# Generated by: TstHWin v2.21b - Registered to VK2TV
# On : 6/28/2007 20:31:21
# UTC: 6/28/2007 10:31:21
>From: M0SCI@GB7COW.#44.GBR.EU
>To : TECH@WW
>
>Can packet and APRS nodes be run on the same aerial? I recently installed
>a APRS IGate and would like to also share the aerial with a packet node,
>can it be done? I was thinking along the lines of a dual port TNC with
>the correct firmware possibly?
The short answer is yet but the longer answer is maybe.
Are the two modes in the same band or in different bands?
If they are in the same band the answer is still maybe, however, you fall into
the same problem as repeater operators who need to transmit and receive
simultaneously with the same antenna. Some form of diplexer will be needed
and, for in-band with close frequency spacing you will certainly need some
form of multi-cavity diplexer.
However, if some occasional receiver desensitisation can be tolerated, and
you can tolerate a minimum of 3dB (50%) power loss on tx, you may get away with a
hybrid constructed from coaxial cable, but success is likely to be quite
limited with this method. This method is also likely to produce serious
transmitter intermodulation products due to insufficient isolation between
the two transmitter PA's. In commercial transmitter combiners, each
transmitter is usually fed through a circulator/isolator and a cavity
filter, to minimise generation of intermodulation products.
Even with separate antennas for each mode some desense will likely be
experienced, especially if the UK 2m band, which only covers 2MHz, is the
band in question.
The amount of desense will depend on three main factors - frequency spacing
of the two systems, transmitter sideband noise from one system appearing at
the other system's receive frequency and, the receivers overload capability
in the presence of a local transmitter. The further apart the two systems
are in frequency, the less likely will be the problems of tx noise and rx
overload. Transmitter sideband noise and receiver overload from the
presence of a nearby transmitter both result in a level of receiver
desensitisation. Rx desense manifests itself this way - the receiver is
satisfactorily receiving a signal, however, when a nearby transmitter is
keyed, the received signal disappears until the "offending" transmitter is
unkeyed.
It's convenient to look at the requirements in terms of decibels. A given
receiver "might" have an overload point of say, -30dBm. That is
one-thousandth of a milliwatt or, one microwatt. One milliwatt is 0dBm. A
ten watt transmitter is the equivalent of 40dBm. That's 70dB or ten million
times the receivers overload point.
The other factor is the receivers sensitivity and the amount of sideband
noise a transmitter produces at the receivers frequency. A typical receiver
might have a sensitivity of 0.3uV or -117dBm. A typical transmitter might
have a sideband noise level at the receiver's frequency of say, -70dBc.
That is 70dB below its carrier power. For our ten watt transmitter with a
power level of 40dBm, sideband noise falling on our receiver frequency is
-30dBm or 1mW. That's 87dB above the sensitivity of our receiver and that
equates to a signal level of 10uV being needed to overcome transmitter
noise. It surprising how few received signals are of this magnitude, unless
they are quite close.
In the case where separate antennas are being used, some of the attenuation
required can be achieved with antennas spacing. Even a pair of vertical
dipoles, with their adjacent ends almost touching, will provide about 30dB
of attenuation. Further spacing increases this figure.
Excess transmitter power or excess sideband noise arriving at a receiver's
input must be filtered out if the receiver is to perform normally. Sideband
noise must be filtered out at the transmitter, and excess power needs to be
filtered out at the receiver.
If your two systems operate in different bands, say 2m and 70cm, a simple
diplexer would permit combining the signals onto a single coax, with
minimum loss. Obviously, the antenna would need to be dual band in this case.
A little more information with regard to frequencies would be beneficial.
I hope my answer to what appears to be a simple question isn't too
daunting, but their isn't a simple amswer to a complex situation.
Cheers ... Ray
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