[UK-CONTEST] CQ WW SSB 2007 - GM0F (long)

[Ian White GM3SEK]

G3SJJ wrote:
>OK, looks like the antenna then.
>
>I removed all the audio leads, DX Doubler and mHam Keyer out of 
>circuit, then used the second coax run directly to the 40m phased array 
>as well as dissing all the 24v relay connections to the phasing relay 
>box. At that stage there was no feedback. I re-connected all antennas 
>and DC lines and everything was still clear. I re-connected the audio 
>cables, but not including the DX Doubler, and the RF feedback was back 
>again. I then wound 2 turns round a clip-on ferrite on the lshort 
>extension lead that goes from the Heil mic mini-jack plug into the mHam 
>Keyer and the feedback was slightly reduced. Winding about 5 turns 
>round a 40mm OD toroid effected a complete cure.
>
>What is puzzling is that the 40m array is about 130m from the shack yet 
>the problem is only on 40m, so I am still convinced there is RF on the 
>various cabling back to the shack, ie 2 coax runs, 2 x 7 core from 
>Sixpak, 7 core rotator (6 core plus 7th core for 24v phasing relay 
>switching.)
>
>Al G0XBV suggests it could be the MP mic socket which isn't grounded 
>and G4IIY suggests it is the Heil headsets picking up RF. Again I can 
>see these could be contributors but why only on one band?
>

I just posted this on the AMPS reflector, about a similar problem:


A very strong possibility is that a coax shield connection has opened up 
somewhere, so you suddenly have RF currents crawling all over your 
system.

Quite likely the equipment that's playing up is only showing the 
symptoms, but is not the root cause. (My money's always on a PL-259.)

The only reliable way that I know to find out is to check over the 
system with a clamp-on RF current meter. Take 10 minutes to throw one 
together, and you'll be a convert! [1-3]

If you find RF currents on the outsides of coax where no currents should 
be, it could be getting out from almost anywhere. The largest current 
readings are not always closest to the fault, so the only way to locate 
the fault is methodically, step  by step.

Start from the bare transceiver and check that with a dummy load - the 
power cord and the outside of the coax patch lead should obviously be 
completely 'dead' to RF. One by one, re-connect the other 999 patch 
leads in your station until the RF current pops up, and you've found the 
fault.

Try it - it really does work.


[1]  Homebrew:
http://www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm

[2] http://www.w8ji.com/building_a_current_meter.htm

[3] In commercial products that I reviewed, the MFJ-854 is good: 
http://www.mfjenterprises.com/products.php?prodid=MFJ-854
Warning: the MFJ-853 is a "Don't Buy" - you can't even trust the meter
needle to move in the right direction!




-- 

73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek