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Traps and advertising claims

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Subject: Traps and advertising claims
From: (Doug Hall)
Date: Mon, 17 Feb 1997 03:46:14 -0500
>The implication is that actual common  brands of  "tribanders" have been
>tested and found to have almost no gain!     Of course, traps are not
>trouble free, and water, dirt daubers, lightning, and high power can cause
>problems.  However, we are talking about basic performance here, and trap
>antennas seem to be capable of meaningful gain. 
>Has anyone studied the performance of standard (Hy-Gain, Mosley) traps  in
>depth, similar to Tony's analysis of KLM trap losses?

   Some months ago I embarked on just such a project. I was trying to make
some sense out of the claims of various antenna manufacturers. I took the
traps from my 3 element trapped tribander to the lab here at work and
measured them. (Hurricane Fran had thoughtfully dropped two very large trees
on my tower's guy wires so it was easy to get the traps from the antenna.
They were just laying on the ground.) I used a Hewlett Packard vector
impedance meter and a Hewlett Packard Q meter to determine the L, C, and Q
values. It's interesting to note, by the way, that the Q of the coils
measures around 70 when they're removed from the aluminum housing.
Calculated Q when the traps are assembled (using the data from the vector-Z
meter) is around 45. This was lower than I expected. I then modelled the
antenna using this data, and according to the model the traps add nearly 3
dB of loss on 20m. Take this with a grain of salt - I can change parameters
slightly and see the loss range from about 2 dB to over 3 dB. I'm not an
antenna expert, so there is certainly room for error here. I do find it hard
to imagine losing 3 dB in the traps - you'd think more people would be
burning up traps with their Alphas. But I think I'm in the ballpark. Has
anyone else tried this, and did you get similar results?
   It's interesting that everyone talks about how much gain antennas have
over a dipole, but few people ever actually compare these antennas to a full
size dipole at the same height. I had a chance to do that last year with an
(admittedly) old TH3 and could see no real difference in received signal
strength. There was considerable F/B, but if the TH3 had any forward gain
over a dipole I couldn't see it. As I said, this was an old antenna, so the
traps could really have been in bad shape. It always seemed to work OK. On
the other hand, a friend back home had a TH3 Jr. and claimed he could nearly
always seen an "S-unit" difference (whatever that is) between it and his
dipole. Go figure.
   We can talk about models and manufacturers claims and theory and how
things "seem" until the cows come home and get tangled in the beverages, but
until we start making some real, meaningful measurements we probably won't
be any closer to the truth.
Doug, KF4KL

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