Adding a coax relay is not hard. I don't get why its such a big deal to power preamps thru the coax. Serious VHFers do not do this but run separate power wiring to the preamp.
BTW you can leave preamps powered all the time (even when transmitting); GasFets are more immune to burn out when biased on. I am talking about a small amount of RF leakage thru the coax relay. This applies only for simple preamps and not the RF switched versions. Most of those sense RF and switch to bypass in transmit. Transmitting directly into a simple preamp will destroy it.
If you are running a transceiver with only a single antenna connector for receive and transmit, you will need two coax relays: one on the antenna side of the preamp and one on the radio side. In transmit the relays bypass the preamp with a short coax jumper. The coax relays will need to be controlled by the radio's PTT control line. Most high-level muli-mode radios have this available on an accessory jack. I do not know if this is normally available on FM-only VHF/UHF radios and you will have to look in the manual for your radio to determine this.
If you must power your preamp thru the coax then DEMI makes a bias inserter. If you do this then you will need to pick off 12v from the inserter to power the preamp and relays. Configure the relays to bypass when unpowered. Again, this does not apply to RF switched preamps. As far as I know most radios are unable to supply sufficient current to power a relay. I think powering preamps with your radio up th ecoax mostly applies to the RF-switched versions.
As a side note: I have added a webpage to show how to provide a direct rx antenna connection on the FT-847: http://www.kl7uw.com/FT-847RXANT.htm
It refers to article done by DL4MEA.
73 Ed - KL7UW
At 06:53 PM 4/19/2008, Michael Pfeuffer wrote:
DEMI makes a couple of nice pre-amps, but they don't have switching and require external power. Has anyone tried to add a T/R switch & coax bias to mast mount them?
--Mike WQ5C
At 03:23 PM 4/18/2008, Michael Tondee wrote:
I just got the Icom AG-35 to use with my IC-820H. It's a bit cheaper than the ARR and SSB electronics units and handles 100 watts. I know that it doesn't have as good a noise figure as the ARR and SSB electronic units but I couldn't afford them and I got the Icom at a discount from a ham store I used to work for. I used to have an ARR 160 rated watt mast mount and sold it when I sold my TS-2000X. I'm just now about to install the AG-35 so I hope it helps. One things for sure, it's tough sledding without a preamp on 70cm. 73, Michael, W4HIJ ----- Original Message ----- From: "Mike Ryan" mryan301@comcast.net To: amsat-bb@amsat.org Sent: Thursday, April 17, 2008 11:13 PM Subject: [amsat-bb] Any feedback on Mirage KP1-440 Preamp?
Hello all,
Zapped my ARR RX only preamp for the second time out of stupidity. Time
for a switched model. Cant afford the $200+ amps from SSB and such. The Mirage seems attractive. Can anyone share their experiences with this model or suggest a differnt affordable preamp? I looked at the ARR switched models already. The high powerd mast mount is too much $$ and the low power one only handles 25 watts. I could easily smoke that one out of stupidity as well (my rig can do 50 watts).
Mike WB1AAT
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