Bruce writes:
> How can I tune the parasitic elements (with an analyzer) when supported
by a permanently grounded (shunt fed, Omega matched) tower?
That sounds like a hard thing to do. When I was playing with parasitic
elements - I found the best way to tune reflectors was to put a broadband
noise on the antenna - get a few hundred feet away (or more if possible)
and use a receiver to find the best null frequency off the back. Based
upon that - you can calculate what adjustment to make to the length to tune
the reflector to the desired frequency.
Tuning directors is harder using this method as the forward gain peak is
hard to measure. I could never convince myself that the director helped
very much.
Tree N6TR
On Wed, Nov 6, 2013 at 6:42 AM, bruce whitney <zuceman@yahoo.com> wrote:
>
> 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
> _________________
> Topband Reflector
> _________________
> Topband Reflector
>
_________________
Topband Reflector
|