Foundations of Amateur Radio

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Manufacturer drivel and antennas ...

Posted: 03 Dec 2016 09:00 AM PST


Foundations of Amateur Radio Let me start completely off-topic today with
a thank you for emails and other expressions of concern regarding the
demise of the bearing last week from my messy desk. I did not loose my
marbles, other than the ball bearings in the disposed item and my sanity is
as intact as it ever was. I was also asked for photos of the messy desk and
as a concession to that I'll use a photo of the ball bearing for the
podcast edition this week. How am I able to produce a photo of the
disposed ball bearing? Truth be told, it's in the bin, the bin is in my
office, but it wasn't emptied last week, since there was so little inside,
so the ball bearing lives - until Tuesday when the bin will surely be
emptied. Now, on to Amateur Radio matters, since that's why I'm here,
though based on your emails, I'm not quite yet sure why you're here.
Yesterday a good friend of mine, who tragically has yet to see the light
and become a licensed Amateur came to me with a non-functioning antenna. He
had purchased a so-called "Ground Independent Monopole" suitable for 380 to
520 MHz. You get no points for guessing that this was to be used for a CB
installation on his vehicle. When you read the accompanying material, this
magical antenna has a 4 dB improvement when compared to a 3 wave whip in
the centre of a metal roof. But then when you look at the foot note, it
talks about a 4 dB improvement over a 1/4 wave whip, but pattern tests only
deliver a 2 to 2.5dB actual gain. I can hear you groaning from here. It
leads me to several observations. As a licensed amateur you should be able
to already spot holes as wide as a semi-trailer in those few statements. As
amateurs we're often dismissive of the CB community, but how can they be
held to account if manufacturers publish what looks to me like drivel of
the highest order. The design itself is curious. There appears to be a
loading coil in the base, the centre of the coax is trimmed to a specified
length and inserted through the coil and electrical continuity exists
between the radiating element and the coax shield. After spending some
time trouble-shooting the installation I determined that the PL259
connector at the end wasn't actually soldered to the coax, so we fixed
that. Using my antenna analyser we trimmed the vertical as specified, a
couple of millimetres at a time, but it wasn't setting the world on fire
with the updated SWR charts I was generating. We stopped trimming when we
got close, since cutting off length is easy, cutting on length not so
much. I then re-read the instructions and queried the length of the
trimmed bit of inner coax that was inserted into the loading coil and found
out that it was about a centimetre too long. Fingers crossed we trimmed
that to length and the SWR chart improved. It still didn't set the world on
fire, but at least the SWR wasn't 8 to 1 on CB Channel 40. Of course I've
urged my friend to get an Amateur License, but that's ultimately their own
choice. What I took away from the experience is that even a very basic
Amateur License like the one I hold is sufficient to understand better what
is going on and to be able to begin the process of trouble-shooting antenna
installations. I thought I understood that this antenna was basically a
vertical dipole, but at the moment I'm not sure and I'm wondering if the
loading coil is actually a matching circuit and I wonder why the coax
shield and the radiator are connected to each other. I'm sure the antenna
is designed with the best intentions and I'm moderately confident that it
works as intended. Now all we need to do is train the marketing department
to talk to the engineering department before publishing their materials.
For me the take-away is two fold. Don't blame a CB-er for their lack of
knowledge, sometimes the manufacturer is to blame. The other take-away is
that with a basic understanding of Amateur Radio you can help your fellow
radio operator. Now, where on my desk is that...
This posting includes a media file:
http://podcasts.itmaze.com.au/founda...teur-radio.mp3