Re: [BYUARC] Antenna farm, proposal #2 (fwd)

[Bryan Peterson]

[accidentally sent this to just Brent so the forward is to get it to the
entire list for information]


---------- Forwarded message ----------
Date: Tue, 22 May 2012 12:40:41 -0600 (MDT)
From: Bryan Peterson <bryan_p...@byu.edu>
To: Brent Smith <Brent...@byu.edu>
Subject: Re: [BYUARC] Antenna farm, proposal #2




On Tue, 22 May 2012, Brent Smith wrote:

> Taking into account Jim's suggestion, we would have.
...
> Diamond says their best repeater antenna is the X510HDM, 2m/70cm, gain
> 2m-8.3dB/70cm-11.7dB, 250 watts, wind rating 90MPH(no ice), height 17.8 ft.,
> fiberglass radome, steel radials and mounting hardware, Type UHF cable
> connection, pre tuned. Three piece antenna.
> HRO price $199.95 or with N connector $219.95

If you purchase a dual band repeater antenna I would suggest that you drop the
separate 70 cm repeater antenna. However, if you have a single antenna for 2m,
as would be necessary with a 17.8' antenna (I doubt you can safely fit two
under the beam), you will need to redesign the 2m antenna connections.

There needs to be a minimum of 58 dB of isolation between the transmitter and
the receiver at the transmitter frequency. That would put you on the ragged
edge of having the transmitter desense the receiver. You also have to make
sure that the transmitter output is at least -160 dB down at the receiver
frequency (i.e., the power at the receiver frequency that the receiver gets
from the transmitter must be less than about 10^(-15) W to avoid having the
transmitter open the squelch on the receiver).

Currently we have four homemade cavities on the receiver that are tuned for 32
dB rejection at 145.33 MHz. That was about all we could get out of them.
Combined with the antenna separation this gave us adequate isolation for the
receiver to work properly.

We also had a little trouble with the transmitter wings having enough amplitude
at 144.73 MHz to open the squelch on the receiver so there are three commercial
cavities on the transmitter tuned to 31 dB rejection at 144.73 MHz. These are
the values necessary when the repeater changed frequencies in April 1998. The
transmitter has been worked on at least twice since then. The cavities on the
transmitter may no longer be necessary but we haven't bothered to check.

Bryan Peterson
bryan_p...@byu.edu