How can I tune the parasitic elements (with an analyzer) when supported by a
permanently grounded (shunt fed, Omega matched) tower?
I read the tuning procedure requiring the tower to be ungrounded? - non
resonant, in order to set the length of the parasitics?
Also - with Tom's analysis, that the parasitics are actually sloped verticals -
by virtue of the horizontal portion ground coupling - wouldn't one be better
off stringing multiple (exactly opposite) counterpoise wires?
________________________________
From: Tom W8JI <w8ji@w8ji.com>
To: Eduardo Araujo <er_araujo@yahoo.com>; Topband <topband@contesting.com>
Sent: Monday, November 4, 2013 7:07 PM
Subject: Re: Topband: Spitfire question
At first I was delighted because I got f/b at ground level of more than 20db
across 25 Kc but surprise came when I
could not notice any improvement in the front gain switching on/off the
elements to medium distances (Brazil) or long
distance like Europe where it was pointed to (and same line of Brazil).
I measured FS at 300 mts at ground level and I only detect 1db maximun change
Reading ON4UN book, It looks like because of its configuration, the main lobe
is shifted upward a bit, so I could
think that because of that I measured very little at ground level but not
noticing any detectable change at
Brazil (3000Km) nor Europe (>10000Km) it really makes me wonder if it is
working at all.>>>>>
That antenna, with 25 radials on the center element, models to have about 3 dB
gain over a single element. That seems good to me for two sloped, close spaced,
bent wire elements for director and reflector.
You can be sure it is sensitive to exactly how you construct it, and how good
your ground below the antenna is. It does not mean we do not have earth loss
just because we do not attach a wire to a radial system near earth, so it
should be highly influenced by ground losses around each sloped element end.
In my model, I used 25 radials 100 feet long on the center element. It had very
good F/B ratio, 23 ohms feed impedance, and 3 dB gain. I used normal ground and
wire losses.
When I look at things, I like to look at something almost perfect and seem if
it all seems logical or sensible. I would never expect much more than 3 dB gain
because of the wire folding, earth losses, and thin long wires for parasitic
elements. After all, we only get 5 dB or so gain over a single element using a
Yagi with full size thick elements when it is placed far above earth. A
typical, real, four square is really about 4 dB over a single element.
Certainly long thin wires near earth that are bent and sloped should not
approach that amount, or the amount of a typical Yagi.
It would be pretty easy to go from 3 dB gain over a single vertical to 1 dB
over a single vertical with any mistakes, and perhaps lose that much just
through dimensional changes or earth problems. You are only 2 dB off, it
usually does not take much to do that.
73 Tom
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Topband Reflector
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Topband Reflector
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