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Re: [TowerTalk] 50 ohm yagis (40M2L) ?

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Subject: Re: [TowerTalk] 50 ohm yagis (40M2L) ?
From: Kevin Normoyle <>
Date: Sun, 18 Jul 2010 16:17:13 -0700
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Don pointed out a mistake in what I said...
Correcting something I said about the ARRL antenna book 40M2L.
I said something wrong about it's swr bandwidth, but it confirms what 
I'm saying anyhow.

I made a mistake about what I said about the ARRL 40M2L-W file.
the W in the file I used...means Wire...I just looked at the element 
diameters and they were narrow.
I was just quickly looking for something that confirmed what I had 
learned when I tried to do a direct 50 ohm 40M2L.
So I looked again.

I should have looked at 240-20H. e looks like it it might be high 
in frequency? (using the ARRL provided 240-20H.YW file in Yagi For 
Windows).... yeah it looks like it needs a hairpin match. With the 
hairpin match,  yeah it has nice swr bandwidth.. I double checked the 
file against the ARRL Antenna book (20th edition) and it matches the 
dimensions for the elements.

SO: the answer is twofold. The ARRL yagis are all optimized potentially 
needing a hairpin match.

I can upload the 240-20H file if anyone wants to see this for themselves.

If you have Yagi for Windows, you can see it's DE is short (it's high in 
frequency) until you compute the hairpin match. I did it at 60'

with the match, yes, it has a nice swr curve like Don pointed out to me, 
WITH the hairpin match.

Looking at the book, they say the antennas are designed for needing a 
hairpin match.
chapter 11 "Matching system: Each antenna is designed with a driven 
element length appropriate for a hairpin type of matching network"

so that makes sense.

Basically, they're confirming what I'm saying.
Unless you're talking moxon or phased or dual driven stuff, or 
off-center feed....I don't think you can do a 50 ohm 40M2L design..i.e. 
you want a lower impedance and a matching thing. (assuming you wan good 
gain/fb/swr bw also)

I would also note that the ARRL 40M2L design needs a LOT of L to match, 
(like my experience). It computes, assuming 18" spacing of .25" rods, 
that it needs 3.24uH, so a hairpin length of 63 inches.

Which also affirms what I was saying about how a "good" 40M2L design 
might need more L than you expect to match, and might exceed the DXE 

I think my advice of going with a long 400 ohm ladderline (shorted) as a 
hairpin to start (start even bigger 6'-7') and finding a match and then 
maybe going the coil route was sound. Yes my 40M2L is a little oddball, 
but I think the issues are similar.

Maybe someone with Yagi for Windows and the ARRL antenna book can 
confirm what I'm saying about the 240-20H design there.
It also makes me wonder if the original poster built to the ARRL specs 
or ???




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