Need for 2M antenna preamp with Icom 9700?, Receive tip for what it's worth
(1) Preamp question: AMSAT newbie here, just having made my first contacts with an Icom 9700 and small yagis on tripods on my deck, fixed in place due to Maine weather. Come Spring, I plan to install the M2 LEO Package on an existing tower, with a rotor. I'm interested primarily in Mode B, and so here is my question. Do I really need a mast-mounted preamp with this set up? I'll be using a 100 foot run of LMR 400, by the way. Right now, with a 2M 4 element yagi and a 30 foot run of LMR 400, I have no trouble copying a lot of signals on receive when satellites are in the one yagi's beamwidth, given the 9700's extraordinary sensitivity and built in preamp. Would a 2M mast mounted preamp be overkill and an unneeded expense?
(2) Receive tip: For what it's worth (and probably old news to most SAT ops, so I apologize in advance)…. When using CW on the 70 cm uplink, I've kept the 9700 on USB while listening on 2M. The SSB filter is almost 2 KHz wider than the CW filter. This has enabled me to listen to Doppler shifted CW signals with little or no RX tuning, hear any adjacent QRM and avoid it, and just have one button to hit when I switch from CW to SSB, transmitting on 70 cm.
Thanks in advance for feedback about the need for a 2M mast mounted preamp.
Wes NA1ME, FN54
On 2020-01-13 08:37, Wes Baden via AMSAT-BB wrote:
given the 9700's extraordinary sensitivity and built in preamp. Would a 2M mast mounted preamp be overkill and an unneeded expense?
Hello Wes,
The built-in preamp doesn't make any sense unless you connect your antenna directly at the rig (inches) or have super-expensive zero-loss coax.
As I outlined a few months ago on this list, a fairly inexpensive LNA at the antenna is the best way to improve your station noise figure.
Once the system noise figure has been set, the coax back to the shack doesn't really matter too much other than wasting a bit of transmit power if you use the same coax for TX and RX.
Original message included here:
If you have a big spool of RG-8X (or even RG-8) and some few dollars for a preamp (LNA), go for the preamp! 100' of LMR400 has a loss of 2.7dB at 70cm. Without a preamp, you now have an instant 2.7dB (or worse) noise figure, even before you get to the radio's front-end, and you've spent 3x on coax.
Yes, RG-8X loss at 70cm is 8.1dB, which means your radio's 100W signal is only ~15W at the antenna, but that's more than enough to reach a satellite before antenna directional gain is accounted for.
One can always make more TX power (and today's radios are overpowered anyway), but one can never recover signals already lost.
--- Zach N0ZGO
participants (2)
-
Wes Baden -
Zach Metzinger