Jason,
For some reason this message bounced when I sent it directly to you so I'll
send it via the Ten Tec reflector. My apologies to the other guys.
Jim
----- Original Message -----
From: "JAMES HANLON" <knjhanlon@msn.com>
To: "Jason Buchanan" <jsb@digistar.com>
Sent: Saturday, March 20, 2004 12:07 PM
Subject: Re: [TenTec] Capture area (was 80 meter loops)
> Jason,
>
> I'll intersperse some comments below in your note to me.
>
> Jim
>
>
> ----- Original Message -----
> From: "Jason Buchanan" <jsb@digistar.com>
> To: <knjhanlon@msn.com>
> Sent: Saturday, March 20, 2004 9:49 AM
> Subject: Re: [TenTec] Capture area (was 80 meter loops)
>
>
> >
> > Thank you thank you thank you for your reply - it is exactly what I was
> > looking for - all the theoretical mathematical formulas do nothing for
> > me - I was looking for practical observations...
> >
> > JAMES HANLON wrote:
> >
> > > An end-fed antenna that is an integral multiple of 1/2 wavelength will
> > > present a very high impedance. An end-fed antenna that is 1/4, 3/4,
> > > 5/4 etc
> > > wavelengths long will present a low impedance more suitable for direct
> > > feed
> > > from your rig. The impedance of a 1/4 wave antenna end-fed against
> > > ground
> > > or a counterpoise as you suggest would be about 35 ohms and purely
> > > resistive, half that of a center-fed half wave. If your rig is
> > > designed to
> > > match a 50 or 75 ohm load, you could increase the length of the
> > > antenna to
> > > about 0.28 wavelength for 50 ohms or to about 0.3 wavelength for 70
> > > ohms.
> > > The antenna would show some series inductive reactance at those
> > > lengths
> > > which could be tuned out with a series capacitor.
> >
> > Excellent. Would that same .28 wavelength value carry for all bands or
> > specific to a single band?
>
> Yes, the same 0.28 wavelength value carries over for all bands. That's a
> number that I "eye-balled" from figure 18A on page 24 of the book that I
> recommended to you.
>
> I'm taking the ARRL antenna modeling course right now, and I do know from
> the course that the impedance at the base of a vertical antenna will vary
> depending upon the number and configuration of the radials in the ground
> plane too. So you might find yourself doing a little bit of cut-and-try
> to get the best possible match for your particular situation. Better yet,
> some type of Antenna Tuner or Transmatch would allow you to adjust the
> feed-point impedance at your rig to whatever impedance your rig likes the
> best. It's generally easier to adjust a couple of knobs on a tuner than
> to run yourself ragged changing the length of a bunch of wires outside,
> and a tuner allows you to change frequency and reestablish your match at
> that new frequency with ease. I use a home-brew tuner with my antenna,
> and I have an SWR meter between the rig and the tuner. I adjust the tuner
> for zero reflected power (SWR = 1.0) looking from the rig into the tuner,
> and my rig is very happy.
> >
> > Is this formula used to calculate the length something fairly simple? I
> > will re-consult my antenna books. I don't remember seeing something
> > long these lines in those books though.
>
> From my ARRL Handbook, the length (feet) of a half-wave in space is
> 492/Freq (mHz) . In words, that's 492 divided by the frequency in
> megahertz. The book goes on to say that the actual length of a half-wave
> antenna will be shorter, depending upon the diameter of the wire used and
> because of the capacitive loading effect of the end insulators. They
> suggest using Length (feet) = 468/Freq(mHz) for wire antennas at
> frequencies up to 30 mHz. Remember that is the length for a HALF
> wavelength. You have to double that to get a full wavelength.
> >
> > Would an antenna cut for 3/4 or 5/4 wavelengths have better performance
> > than the 1/4 wavelength antenna (more wire, more capture area)? Or
> > should I stick with the 1/4 wavelength antenna and raise it higher in
> > the air instead...
>
> A vertical antenna longer than 5/8 wavelengths starts to develop some
> appreciable lobes at high angles of radiation. That means that the power
> that you would radiate into the low angles where the DX propagation occurs
> would be shifted into the higher angles where DX propagation does not
> occur. If you were primarily interested in short-distance skip
> communication, say a few hundred to perhaps a maximum of 100 miles, higher
> angle radiation would do better for you because the high-angle signals
> reflected from the ionosphere will come back to earth at those shorter
> distances. If you are interested in longer distance propagation, stick
> with an antenna no taller than 5/8 wavelength that optimizes the low-angle
> lobe.
>
> There is no particular advantage in raising an HF vertical higher in the
> air unless it is surrounded by a lot of other structures that might absorb
> its signal or modify its pattern. You would also have to support its
> radial structure when you lifted it up, which could become a mechanical
> problem. You can just lay out radials on the ground or even bury them
> slightly for a ground-mounted vertical.
> >
> > > A vertical quarter-wave will have a reasonably low angle of radiation
> > > and
> > > will be good for low-angle propagation usually associated with longer
> > > distance contacts. Increasing the height of a vertical fed against
> > > ground
> > > up to 5/8 wavelength will further enhance the low-angle radiation.
> > > Above
> > > 5/8 wavelength height, a second, high-angle lobe develops and the
> > > low-angle
> > > radiation suffers.
> >
> > Found that in my ARRL antenna handbook. Do you measure the antenna's
> > height above ground from the base or from the top most elevated point of
> > the antenna?
>
> I suppose ground level is the most common reference. So for 7.1 mHz for
> example and using the L = 468/Freq formula for a half-wave as given above,
> the bottom of a quarter wave vertical antenna would be at 0 feet and the
> top would be at 32.96 feet above ground.
> >
> > > A good reference book on verticals that you may be able to find is The
> > > Amateur Radio Vertical Antenna Handbook by Capt. Paul H. Lee. It is
> > > published by CQ Communications, Inc, 76 North Broadway, Hickville, NY
> > > 11801.
> > > It is a paper-back and not expensive.
> >
> > Great - I will try to get a copy.
> >
> >
> > Thanks very much for the help and explanations!
> >
> >
> > Jason
> >
>
>
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