I have a general question regarding the pro's and con's of two types of
antennas to be used for fixed station operation from a motorhome.


Is there any efficiency advantage between using a super sized Texas
bugcatcher type center loaded antenna about 18 feet tall, vs, a vertical
pole about 23' tall that is tuned with either a manual, or automatic
antenna tuner?

Either antenna would be mounted in same location, and use my best
connection to the under-chassis as ground point for feed.

My primary operations are on 80M and 40M. Vertical is mandatory due to
certain limited space parking situations.


Ed K7AAT

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

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. 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.

Walt, W2DU

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

Walt, W2DU



Well, I was considering constructions a rather large coil that
should have a Q of 1000, or so..... but I see your point, anyway.
Thank you, Cecil and Denny.


I am tending toward a vertical with a Tuner, now.


Ed K7AAT

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

The devil is in the details... Note that I included the words, "decent
top hat"...
For 40 it should pull the vertical close to resonance...
For 80 I would not argue with your prescription for an outboard
loading coil to help the tuner efficiency...

BTW Ed, a coil Q of 1000 is going to be very tough to hit... If you
do get it please share the details...

denny / k8do

The devil is in the details... Note that I included the words, "decent
top hat"...
For 40 it should pull the vertical close to resonance...
For 80 I would not argue with your prescription for an outboard
loading coil to help the tuner efficiency...

BTW Ed, a coil Q of 1000 is going to be very tough to hit... If you
do get it please share the details...



Well, I may have stretched the Q factor a bit.... but in looking at
the following site, I would suppose its possible....

http://www.texasbugcatcher.com/cata/tbcspec.htm#6inch

scroll down until you view the #680 coil and read his text below.
That is what I had in mind if building a center loaded vertical. I
would not use a top hat in my case.

Per other comments in this thread, I see that perhaps I may get best
performance by using a combination of center loaded for 4MHz and using a
tuner for 40... and maybe 60. I'm really not interested in any other
bands.

As for radials, remember, even though intended for fixed station
use only, this is still a mobile antenna, and my RV parking situation
in many crowded spaces will preclude the layout of radials of any
significance.


Thanks for all the "food for thought" on this.


Ed K7AAT

On Feb 23, 3:11 am, Ian White GM3SEK wrote:


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?

I was kind of thinking the same way.. In general, I would usually
prefer
the larger coil, even with it's problems.
I think a larger part is the top loading, if used.. If no top loading
were used
for the 24 ft whip, I could see it being a toss if the loading coil in
the tuner
was small and rinky dink. It could easily lose to the center fed if
the tuner
is poor.
But then you also should consider current distribution through the
whip.
Can make quite a difference. If the 24 ft whip was fed with a tuner,
and
no hat was used, max current will be at the tuner coil.. Not really a
good thing.
But adding the hat will greatly improve current distribution through
the
whip, and pretty much cure that problem, assuming the hat is large
enough.
If no hat is used, I would always prefer a center loading coil. Mainly
for
current distribution reasons. But also cuz many tuners are overly
lossy
when used in such a manner. Note comparisons with a tuner fed whip on
a
car, vs a good center load antenna. In the average case, the tuner
fed
whip will be about 12 db down according to the mobile "shootouts".
As far as coil Q, 1000 is going to be pretty optimistic... 300 + -
would be
more easily obtainable.. And thats a pretty decent coil.
If ground radials are ignored, ground loss could easily swamp the coil
loss.
Of the two losses, I'd be more worried about ground, than coil loss.
But thats just my 29 cents worth...
MK