> This year I got up at the 87 foot level a 4 element modified
> A-4 on a longer boom of 35 feet. From what I have able to
> determine with the new retuning of the cage feed is that the
> stinger is no longer visible at RF. I determined, correct me
> if I am wrong, that since the resonant frequency decreased
> and the base impedance impedance increased on my MFJ-259B
> that the stinger had vanished because of the large yagi
> below it....
Somewhere in the distant past I heard or read a rule of thumb
that said the top loading contribution of a yagi was about 1.8
times the turning radius it the antenna was relatively "square."
That is the element length was about the same as the boom
length. The 35 foot boom modified A4 fits that criteria with
a 35' boom and elements (trapped) about 30' long. That makes
the turning radius of the antenna about 23'
A quick check with simplified models in EZNEC shows:
85' tower 2.75 MHz (85')
85' tower with 35' mast 2.06 MHz (113')
85' tower with modified A4 1.69 MHz (138')
85' tower with A4 and mast 1.54 MHz (152')
The "loading contribution" of the yagi is around 48' ...
more than the "stinger's" 28' (shorter than its physical length
due to the significantly reduced diameter). However, the
stinger still provides some additional loading. It is
interesting that the yagi provides top loading equivalent
to about two times its assumed turning radius ...
relatively close to the old 1.8 x turning radius rule of
thumb.
The simplified models assume uniform cross section tower (6"
diameter) a 2" diameter mast, and only the first/last elements
of the yagi (with no traps). They are instructive for trends
... resonant frequencies are probably not good within +/- 200KHz.
73,
... Joe, W4TV
> -----Original Message-----
> From: Herbert Schoenbohm [mailto:herbs@vitelcom.net]
> Sent: Tuesday, November 10, 2009 7:29 PM
> To: lists@subich.com
> Cc: 'Jon Zaimes AA1K'; 'Topband'
> Subject: Re: Topband: Electrical height of short shunt fed tower?
>
>
>
>
> Fascinating information on stacked antennas for a shunt fed
> tower but.......
>
> in my case I lost my A4 Tribander after Hurricane Omar but
> the 85 foot
> Rohn 45 remained intact. So I ran up a 35 foot aluminum
> stinger on the
> top and retuned everything. It worked as expect although not quite a
> 1/4 wave. This year I got up at the 87 foot level a 4 element
> modified
> A-4 on a longer boom of 35 feet. From what I have able to determine
> with the new retuning of the cage feed is that the stinger is
> no longer
> visible at RF. I determined, correct me if I am wrong, that since the
> resonant frequency decreased and the base impedance
> impedance increased
> on my MFJ-259B that the stinger had vanished because of the
> large yagi
> below it.... I wonder if anyone has checked this out with EZNEC that
> the only plausible reason for a stinger above a large yagi
> below is for
> a longer lightning rod at the top of the tower.
>
> I think a good rule to apply is that 1/4 physical height on
> 160 is not
> required for a 75-90 foot tower with a large yagi on top. But
> this may
> be wishful thinking but I would like to think it is true.
> Mostly likely
> the same principle applies to Marconi "T"s with at least 75 foot
> vertical drop and enough balanced horizontal to make the
> antenna appear
> to be a 1/4 wave at RF and you have a great TB antenna assuming
> reasonable amounts of radials.
>
>
> Any thoughts?
>
>
> Herb Schoenbohm, KV4FZ
_______________________________________________
160 meters is a serious band, it should be treated with respect. - TF4M
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