Walter Maxwell wrote:
On 22 Feb 2007 05:09:27 -0800, "Denny" wrote:



Ed
With a decent top hat, the bottom fed 24 foot will outplay the shorter
antenna... For this installation the SGC auto tuner should work
satisfactorily... I would go this route over the Bug Catcher,
myself...

Consider some roll out radials clamped to the ground tap on the
antenna feed for when you are able to lay wires across the ground -
like after midnight... This will make a major improvement...

denny

Hi Ed,

I agree with Denny 100%. Any loss in a tuner feeding the 24 ft vertical will be
less than that in the Bug Catcher.

I'm not so sure. A 24ft vertical is about 0.18 wavelengths at 7MHz, and
about 0.09wl at 3.5MHz. It will require a large series inductance to
resonate it at 3.5MHz, and will also require some series inductance for
7MHz.

That inductance must be provided by either an external loading coil or
by the auto-tuner. The question is: which will have the smaller loss, a
large and well designed external coil, or the tiddly little toroids
inside an auto-tuner?

The major loss in the Bug Catcher is due to
the resistance caused by the turn-to-turn capacitance that makes the inductance
resonant at some frequency not too much higher than that that at the operating
frequency. See the resistance vs frequency curve of a parallel LC
circuit, where
you'll see that the value of resistance is maximum at resonance, which is not
what you want at anywhere near the operating frequency.

Fair enough, but a loading coil doesn't always have to be built that
way. This one has no requirement to catch bugs at 60mph.

I would suggest the 24ft vertical and auto-tuner by all means, but use a
well constructed outboard coil to provide the major part of the loading,
and leave the auto-tuner to handle whatever impedances result.

Without analysing it in detail, it would probably be best to load the
whole thing to resonate just above 4MHz. At 7MHz the feedpoint impedance
would then be considerably inductive, but the tuner can probably deal
with that more efficiently than if it was asked to provide all the
loading inductance itself.

Another alternative, avoiding the auto-tuner completely and therefore
much less expensive, would be EI7BA's homebrew version of the Butternut
HF2:
http://ireland.iol.ie/~bravo/80&40mVertic.htm


But whatever you choose, it's got to have "radials, radials, radials",
exactly as Denny says.

In your situation you can never come anywhere near the optimum number
and length of radials, so simply aim for as many and as long as you can.
On the ground or just below the surface, whatever lengths you can
practically manage will be fine.


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek