[Elecraft] Astron RS-35M - Problem Related to RF !!!

Thu Mar 18 14:01:01 2004

I'd bet on RFI in the speaker amps - it doesn't take much to clobber those
things.  I use a Heil SS-2 amplified speaker with my K2, and it sounds
absolutely great, but you do have to be careful about RF getting into it,
even thought it already has some protection. 
Moreover, this whole thread on RFI and Astron is curious. I've used a VS-35M
for 10 years to power a dozen different radios and PA's over that time,
including by Kenwood, Icom, Yaesu, Ten Tec, Mirage, and K2 on HF, VHF, and
UHF. Most often, a kilowatt HF linear was in line with the radio,
occasionally, an 80W brick or a 400W tube PA on 2M.  These days, it's used
only for the K2 plus a 2M FM rig and a 10M all mode. 
The Astron has never as much as blinked, UNTIL two days ago.  I was using it
to power my K2, and I keyed up my Omni V, which was set at minimum RF O/P
(10W), using a different PS, on a different band, and the antennas are 70'
apart.  The Astron crowbar folded over!!  I get the impression that it's
more than coincidence, but I'm not ready to make any judgments.
Cheers,
George - K1GXT

-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Mike McCoy
Sent: Thursday, March 18, 2004 11:23 AM
To: [email protected]
Cc: [email protected]
Subject: RE: [Elecraft] Astron RS-35M - Problem Related to RF !!!

> No doubt you can change the behavior - even stop the shut-down -
> by messing with the antenna, but antennas do not "cause" these
> issues.

OK, I'm going to demonstrate my ignorance here but I'm curious about the
phrase that 'antennas do not cause these issues'

I presently have an RFI related issue with a 87' W9INN MPD5 (80-10) dipole I
just hung at about 30' across the roof a few days ago. Not with the PS but
with a 'hum' emanating from my (powered) PC speakers when I xmit. This 'hum'
occurs ONLY on 20 mtrs.

We're talking a measly 5-10w out here (no K2/100)

Due to (I suspect) a capacitive effect resulting from a close the proximity
to the roof the 20 mtr element is currently resonant around 13.8 Mhz and
thus has a fairly high SWR even at the bottom of 20 mtrs.  The KAT2 easily
handles the mismatch but of course a standing wave current is still present
on the feedline.  Of course this is unacceptable and I will re-tune the
antenna but in the meantime...

"All things being equal" I also have a Sigma GT-5 that is actually
physically closer to the rig/speakers than the dipole but does NOT cause the
speakers to hum when xmitting on 20.  I presume I have no RFI issue with the
GT5 due to an excellent 'in band' match.

Since 'one antenna does and one antenna doesn't' it sure seems to me the
(dipole) antenna is causing an RFI issue.  The coax that is likely actually
radiating the RFI (?) is just a symptom, the effect of a badly tuned antenna
and the antenna is the real source of the issue.  If not the antenna then
what?

Reason I ask is I am *assuming* (and we all know what that means) that since
I didn't have RFI problems prior to this once I get the antenna properly
tuned the RFI will go away.

p.s. This should in no way reflect badly on W9INN antennas. They are really
great. I bought it used and since they are individually custom built I am
simply having to 'tweek' it to my particular circumstance/installation.

Mike 'hope to learn something here' K5PU

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