He has already got a match so thats not the issue. The bandwidth appears to
be poor with the L network he is now using. Variable capacitors are high Q
and will narrow the BW.
Since the tower is electrically short it properly will need inductive
reactance to match it as he has already done but the capacitor is not
necessary.
The tower is already insulated from the ground so matching is very simple.
Place a coil from the base to the ground system and find a feedline tap
point that is 50 Ohms as noted on a Bird 43 or whatever. Use 1/4" copper
tubing for the coil and the interconnections for lowest loss
If the BW is still too narrow then configuring as a folded unipole is one
method to widen it. The feed impedence will be much higher resulting in less
matching loss. Calculate the impedence as if you were doing a folded dipole
or all metal T match.
Carl
KM1H
----- Original Message -----
From: <john@kk9a.com>
To: <TOWERTALK@contesting.com>
Sent: Friday, September 28, 2007 4:37 PM
Subject: Re: [TowerTalk] 160m 1/4 Wave Vertical Results Thus Far...
> You should should be about to get and SWR of less than 1.7:1 with no
> matching. Adding more radials will not help your problem. This is
> normally
> a simple antenna and of if an SWR of 1:1 is desired you can use a simple
> hairpin coil to match the impedance. You clearly have something wrong.
> Perhaps your coax, a connector, your SWR meter is bad or maybe it has
> something to do with your base being four feet off the ground. I have
> build
> a number of 1/4 wl verticals using direct feed with great results, however
> my base was just inches above the ground.
>
> John KK9A
>
> To: towertalk@contesting.com
> Subject: [TowerTalk] 160m 1/4 Wave Vertical Results Thus Far...
> From: AB5MM
> Date: Thu, 27 Sep 2007 19:51:38 -0500
> List-post: <mailto:towertalk@contesting.com>
>
> Gentlemen,
>
> I would like to extend a hardy "Thanks" to each of you that responded to
> my plea for help with matching the 120'-6" 160m Rohn 25 Vertical. It's
> still not working like I think it should, so let me tell you what we've
> managed to accomplish to date.
>
> 1. We now have 40ea. 125' long ground radials installed. Another 20 will
> be added this weekend for a total of 60. I think I'm going to say that's
> enough for now. NOTE: Going from 22 to 40 ground radials made little
> difference!
>
> 2. Both the MFJ-249 and MFJ-259B are virtually useless at these
> frequencies. Too much rf from the BC band. This alone would have saved
> us hours in trying various matching networks, not to mention the number
> of "Pepcid Complete" ingested.
>
> 3. In sweeping the tower from 1.600 MHz to 2.500 MHz, we found it's
> natural (no matching other than the 75' of LDF4-50A) resonance at 1.788
> MHz. The lowest swr observed at this frequency was 2.6/1. At 1.800 it is
> 3.0/1 and slowly rising to 4.0/1 at 2.000 Mhz. Conventional wisdom tells
> us that the thing is too long (tall). This still brings up the
> question, "How can this be the case when the vertical is only 120 ft.-6
> in. tall to begin with?" What are we missing here guys? The center of
> the band (1.900 mhz) is what we've been shooting for all along.
>
> 4. A high pass "T" network brings the match down flat, but it's Q is
> high enough to only realize a practical band width of ~33kc.
>
> 5. A low pass "L" network works as well and exhibits a little larger
> band width of ~60 kc. Our goal is ~100 kc wide... is this reasonable?
>
> We really don't know what to do or try next, if anything. Should we be
> satisfied with what we have?
>
> Press on, nothing is as persistent as persistence itself.
>
> Tnx & 73,
> Steve AB5MM
> Lloyd K5ZO
>
> _______________________________________________
>
>
>
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