This is not meant to suggest that it is not a compromise antenna, but I use the
Hy-Gain AV640, which is a 3/8 wave radiator (either full (electrical) size, or
shortened by a capacity hat, or one coil and capacity hat, depending on the
band) vertical. I had often described it as a vertical OCF dipole, as its
feedpoint impedance is about 200 ohms, and it somewhat parallels the feeding of
an OCF. In conversation with Hy-Gain's Tom Stone, he described it as a
vertical windom; not sure how much difference there is.
In this case, though, rather than 1/8 wave radials cut for each band there is
simply the set of counterpoises, which supposedly are adequate against the 200
ohm feed point. The feed is run vertically up to the matching unit with a
choke and 4:1 unun in it.
I will say this: it's about 18' feet at the feed and it's rare that on 30 or 40
(20 usually closed by the time I get home from work) that I won't quickly break
any pile up I hear.
73,
Art, KØRO
--- On Wed, 11/18/09, Kelly Taylor <theroadtrip@mts.net> wrote:
From: Kelly Taylor <theroadtrip@mts.net>
Subject: Re: [TowerTalk] Vertical dipoles
To: "Steve Hunt" <steve@karinya.net>
Cc: towertalk@contesting.com
Date: Wednesday, November 18, 2009, 8:42 PM
Hi all,
Shortly before we lost him all too early, I sent L.B. Cebik a note
describing an idea I had for a vertical dipole.
While he didn't go in to great detail, he did offer an excellent suggestion:
feed it at the intersection of the lower loading arm (whether it comes off
as a T-bar or the lower section of an L) and the vertical radiator.
Basically, his idea was to transition the idea of an OCF dipole to a
shortened vertical dipole.
The trick is determining feedpoint impedence and matching to it. But if you
feed it properly, you should be able to set up the same current patterns as
a regularly fed vertical dipole without the nasty business of having to
dress the feedline away from the feedpoint at a 90-degree (or close to
90-degree) angle.
Kelly
ve4xt
On 11/18/09 3:25 PM, "Steve Hunt" <steve@karinya.net> wrote:
> It models fine for me:
>
> 120ft vertical radiator
> Bottom "capacity hat" 8ft off ground, 4 x 20ft horizontal wires + 4
> perimeter wires
> Two top loading wires at 45 degrees, each 28ft long.
> All wires #14 copper over average ground
>
> Fed in the centre of the vertical radiator, at 1.93MHz it has a
> feedpoint impedance of 72+j2 ohms
>
> Dimensions taken from the QEX article.
>
> 73,
> Steve G3TXQ
>
> K4SAV wrote:
>> EI7BA wrote: Yes I am I am talking dipoles here..The dipoles have single
>> wire
>> capacity hats at either end, which is the most efficient way of loading
>> them. The bottom capacity hat wire is 10ft over ground, and the top hat
>> is at 55ft. all the details are at
>> http://www.iol.ie/~bravo/low_band_antennae.htm#My%20TX%20Antennae
>>
>>
>> The antenna you are describing on that page is commonly called a top
>> loaded T, with two elevated radials (not a vertical dipole), although
>> the radials described are only 1/8 wavelength each. There must be some
>> mistake on that diagram because when I put those dimensions into EZNEC I
>> get a resonant frequency of 2.43 MHz. I don't get any of the other
>> characteristics described on that page either. The antenna was
>> described as having a feedpoint impedance of 65 ohms and a 2:1 SWR of
>> 170 KHz, and a gain of -0.25 dB. I get a feedpoint impedance at 1.85
>> MHz of 10.4 -j278 over average ground. Bandwidth at that frequency has
>> no meaning.
>>
>> I tried to find the error. I can make the antenna resonate on 1.87 MHz
>> by lengthening the two radials to 133 ft each, but the feedpoint
>> impedance goes to approximately 12 ohms over average ground. It's even
>> less over salt water. I could make the two top wires 130 ft each
>> (instead of 65 ft each) and make the antenna resonate on 1.835 MHz, but
>> the feedpoint impedance goes to approximately 17 ohms over average
>> ground. I could simulated a feedline that is not decoupled from the
>> antenna and causes a lot of ground loss and that would raise the
>> impedance and widen the bandwidth. The gain is largely dependent on
>> whether you are talking about average ground or salt water. So does
>> that diagram accurately reflect what you built?
>>
>> Jerry, K4SAV
>>
>> _______________________________________________
>>
>>
>>
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>>
>
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