I concur, he should definitely try measuring the antenna as close as
possible to the base.
----- Original Message -----
From: "jeremy-ca" <km1h@jeremy.mv.com>
To: <john@kk9a.com>; <TOWERTALK@contesting.com>
Sent: Friday, September 28, 2007 11:20 PM
Subject: Re: [TowerTalk] 160m 1/4 Wave Vertical Results Thus Far...
He hasnt determined any reasonance yet, just some readings that are easy to
misinterpet by those not well versed in that area.
And as already mentioned, you do not measure an antennas characteristics at
the end of 75' of coax and expect anything meaningful unless the load is a
pure resistance. Or have plotted on a Smith chart and know how to interpet.
Or/and have some professional test equipment on site.
Carl
KM1H
----- Original Message -----
From: <john@kk9a.com>
To: "jeremy-ca" <km1h@jeremy.mv.com>; <TOWERTALK@contesting.com>
Sent: Friday, September 28, 2007 10:15 PM
Subject: Re: [TowerTalk] 160m 1/4 Wave Vertical Results Thus Far...
>I agree that the tower is too short, however when he measures it, it is
> electrically too long and has an SWR of 2.6:1. Something is very wrong
> and
> adding an inductor won't help until the problem is resolved. If it were
> me,
> I would either add a stinger or a short conductive guy wire at the top to
> bring it to resonance, instead of using base loading.
>
> Does your coax run along the ground with the radials?
>
> John KK9A
>
>
> ----- Original Message -----
> From: "jeremy-ca" <km1h@jeremy.mv.com>
> To: <john@kk9a.com>; <TOWERTALK@contesting.com>
> Sent: Friday, September 28, 2007 5:52 PM
> Subject: Re: [TowerTalk] 160m 1/4 Wave Vertical Results Thus Far...
>
>
> 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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>
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