Astro Spectra Zapped CR3903 <again>

[Tim]

Hi Folks,

Figured I'd ask around... The Spectra had poor sensitivity yesterday (5uV), and I tracked it down to a shorted CR3903 (switching diode on the PA - feeds RX).

I replaced it - all was well, and then this afternoon I noticed the sensitivity had gone to pot again. Yep, 5uV.

My setup - VHF 5/8wave on top of Yukon in front. Base loaded low band antenna on top in the back (separated about 4 feet).

Is there a problem in having the two antennas that close together? I never had any problems when the Spectra was an X9000. Have seen lots of public safety vehicles with similar configurations.

Also had the same problem in my wife's Impala with the 2 antennas on the back deck. Replaced the VHF with a 1/4 wave & hasn't had a problem. (I think that the two antennas had crossed at some point while talking on 6 meters).

Anyway, any ideas?

Thanks,

Tim

[AL7OC]

Your 5/8 high band antenna is close to being an electrical 1/4 wave on 6 meters. You will couple a lot of energy into the 5/8 high band while talking on low band, which is probably taking out your front end on the high band radio. You can go to a quarter wave on high band to reduce the problem, or place a low pass filter in series with it. If you can find a dlplexer with a port for 6m and a port for 2m, put a small 50 ohm termination on the 6m port, your high band to the 2m port, and your 5/8 VHF antenna to the antenna port. Any coupled energy from the 6 meter radio will be shunted to the terminator, and protect your highband front end. You can repeat the exercise on the low band radio using appropriate ports to protect the low band front end.

Two downsides to this - if you are coupling enough energy to damage your radio, then the 5/8 VHF antenna is probably messing up the radiation pattern of the low band antenna and visa versa.

Also, diplexers have some small insertion loss.

There may be some optimal near field spacing for the two antennas to reduce coupling, but I don't have that off hand. You would have to experiment with the antennas on a ground plane and measure the coupled energy to see if there is a separation that minimizes coupling.

Good luck.

[Will]

AL7OC IS correct. Either excess coupling between antennas and or high VSWR on the VHF antenna.

Next you will probably blow out the receiver frontend in the Spectra.

[Jim202]

Have you thought about putting a couple of diodes back to back in parallel to try to cut down on the voltage getting to the switching diodes and front end. Just put a resistor of say 56 Ohms in series with the input RX path and on the RX side, put the diodes from that point to ground.

Had to do this on a GE Delta-SX radio that a UHF radar was blowing the front end FET up. I used some hot carrier diodes that I had.

Jim

[Will]

I am assuming a hi power VHF radio. Did you replace both CR3903 and 3902?

And you checked the dc bias on CR3903 in TX?

[Tim]

Thanks all for the inputs.

I put the service monitor on the VHF antenna, and it looks like about 7 watts is being coupled from the LB to the Spectra through the 5/8 wave antenna.

I didn't replace CR3902, as it wasn't shorted, but I didn't check it for reverse. Perhaps it's open & not doing anything! I'll check it when I pull the amp off the heatsink to replace CR3903. If 3903 is shorted, but 3902 isn't, that's really kind of odd, as 3902 is the 'first line' of defense in the receive train.

First thing I'll do is put the service monitor on the VHF antenna, but make it a 1/4 wave just to see what kind of coupling I'll get into it.

Next, I've got a spring on the side of the truck in the back for a HF antenna... I might try a 1/4 wave LB antenna & check the coupling there again. Would probably give me an odd pattern, but I can't say that the loaded 1/4 wave on the roof is any good either!

Thanks. I'll report back once I get the tests done tomorrow.

Tim

[Tim]

Replaced the 5/8 wave with a 1/4 wave, and the power just barely registered on the service monitor.

Did check CR3902 & voltage, and all was well. Funny 3902 didn't do a better job of protecting the downstream components.

Ended up making a 3 pole high pass filter in a shielded box. Lose about 4 watts in TX, negligible in RX. With the 5/8 wave on the top, the power just barely registers in the service monitor with the filter in line.

Guess we'll see how it works.

Thanks,

Tim