[AMRadio] Dealing with Ice on BC Antennas

[John Lyles]

When I was doing engineering at WUVT at Va Tech in the late 1970s, we 
had a composite FM transmitter, a Bauer 660 exciter driving a homebrew 
4CX300A class C amplifier. I learned a lot about icing and tetrodes 
during those few years, as the loading on the antenna would typically 
drive the screen current all the way up to trip point, in this case 
blowing a fuse in the power supply. Screen current is a very sensitive 
indicator of loading on a tetrode. My solution back then was to go up to 
the 7th floor rooftop, and whack the mast with a 2 x 4 timber, knocking 
the ice off.

Later, when designing FM transmitters for Broadcast Electronics, that 
experience was remembered. BE, Collins (now Continental) and Harris all 
used tetrodes, while there were some others like CCA, Elcom Bauer, CSI, 
etc who used GG triodes. Our automatic power controller was simply a 
servo control on the screen power supply primary AC. This would lead to 
trouble with a varying load as the transmitter could run into 
overcurrent on G2 if not careful. It wasn't a big stretch to design 
another feedback path from the reflected power monitor on the rig. This 
became VSWR foldback, and of course, we weren't the first to do it as 
military and amateur solid state rigs had some rudimentary protection 
like this as well.

So the BE FM3.5, 5 and 30 kW at the time had this new feature. We heard 
from one station in New Orleans area where a freak ice storm covered 
their antennas (no radomes used down there). All their competitors were 
tripped off the air, but their 30 kW transmitter just dialed itself back 
to a few KW, melted the coating on the elements and came back up to 
power, without loosing airtime. We heard feedback from numerous 
customers over the winters with similar results.
John
K5PRO


> Message: 3
> Date: Mon, 16 Dec 2013 12:11:01 -0600
> Back in the Seventies, I worked as assistant chief at a 27 KW FM stereo
> station in western Missouri.  The antenna we used was a 5-bay
> circularly-polarized unit fed with 3" hardline from a 10 KW
> transmitter.  The antenna was mounted on top of a 60-foot Rohn tower on
> the roof of the Administration Building.  We had fiberglass radomes
> around the elements, but no deicers.  Since we were in MO, we frequently
> had freezing rain or other precip which would build up on the radomes,
> and detune us.  To get rid of the ice, we would retune the transmitter
> and heat up the radomes.  Pretty soon we would start hearing 'thumps' as
> the ice started coming off, and then the power would shoot back up.  We
> would retune the TX back where it belonged, and everything would be
> great till the next ice storm.  We were fortunate that we were putting
> out 10 KW with an RCA BTF-10E transmitter, which used a 4CX10,000D PA
> tube.  That big guy never even noticed the mismatch!  I wouldn't want to
> try that 'trick' with a TX using a tube  operating way over its plate
> dissipation rating, the way a lot of tube-type ham radio amps do.
>
> Mike, WB0LDJ
> Former First Phone holder - now General :(
>