Here is a super 2 element 40 meter Yagi. All of the hard design work has
been completed. The feed system is even easier - via 50 ohm coax (suitable
choke).
I have two of these in the air and they work perfect,
http://www.k3lr.com/engineering/moxon/W6NL_Moxon104.pdf
73,
Tim K3LR
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of
knormoyle@surfnetusa.com
Sent: Saturday, July 17, 2010 11:51 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Beta match adjustment
Scott said "I built a 40m 2 element Yagi. Physical detail on the antenna
can be shared
if pertinent. It plays well, but I would like to utilize a DXE beta match
to see if I could improve the match.
I have an antenna analyzer thus can measure a variety of antenna parameters.
At my frequency of interest, R=Z @ 26 ohms, and an angle of 0 degrees. This
is without the Beta in place. Beta in place has yielded no worthwhile
results, regardless of setting."
Funny, I just completed something similar. I tweaked mine with the MFJ-259,
while up at about 25' on a
crankup.
I had an old 40M2L (linear loaded 40M shorty) sitting around, and thought
I'd convert it to a moxon.
Then I didn't like the sag, so after I had it up, I said, "Hey I can just
have a bent tip yagi" and
let the bent tips hang vertical. I did a bunch of NEC simulations, liked
the results, so did it.
So I was using a beta match too..the DXE, and here's the thing:
All this talk about 25 or 26 ohms is not backed by anything, right? You
can't say what the optimal
feedpoint impedance is for your antenna design, without more info? It might
be lower, it might be
higher.
Unless you simulated your antenna you don't know the optimimum feed
impedance, for good f/b/gain/swr.
Right? I had a little bit short boom...and while we know most yagis are
historically targeted around
the 25-28 number, it's not gospel?
SO: that all said: there is some set of element lengths, both DE and
reflector, boom spacing, and L
for the hairpin, that combine to give the best tradeoff for your antenna.
I was surprised by how far off my simulation was. I had to had something
between 12" and 24" to the
tips. But I'm a NEC newbie (although it was a lot of fun..I included the
hairpin match in the sim)
SO: what am I getting to: I had a ladder setup so I could play without
taking the antenna up and down
(it was on a crankup)
You didn't say how you measured your 26 ohms. If it was at the end of some
random length of coax, that
doesn't mean much, right?
Back to the DXE: Surprisingly, for 40m and this yagi, the DEX hairpin just
didn't have enough L. I
tried widening it with different bars..but it was just a little bit not
enough.
So I cut a 4-5' piece of 400 ohm ladderline (shorted ends) that I used as a
hairpin..just left it
hanging..and trimmed that (while adjusting the tips and making measurements)
Then I took it off and measured the L, and did a tubing coil (about 8
turns...3" diameter or so..I
used copper..couldn't find the right aluminum)..I then jiggered with the
coil spacing to get a good
match again.
The end result was pretty good. I get <2:1 Swr over the full 7-7.3Mhz...And
I can do A/B comparisons
against a xm240 I have up that's below a 20M 5L, a little higher
crankup..and that's not as tuned in
(not much F/B)...so far, I think there is benefit to this antenna, for me.
So in summary:
-If the antenna was homebrew, you probably want some amount of simulation
with proper taper schedules?
-You might need more L than the DXE hairpin can provide at 40M
-a length of ladderline is useful for seeing that you can effect things..you
can make it really long
(4-6')..
-this was the first time I used a coil for the beta match. I liked it..
Now as to whether a bent-tip yagi is just nuts. well maybe. :)
-kevin
ad6z
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