Bob, I want to modify slightly: 468/f gives us the APPROXIMATE resonant
length. NEVER CUT FOR THIS LENGTH.
Always cut a little longer, fold back the ends to adjust to this calculated
length - then measure SWR. Better yet, measure for true resonance (good
analyzers show it).
The antenna is in resonance at the length where the antenna is purely
resistive, having no inductive or capacitive reactance. This is almost
never the point of minimum SWR (though usually close).
Being in resonance has absolutely nothing to do with 50 Ohms.
As you correctly stated, the impedance at resonance will vary depending on
height and other factors.
If you then adjust the length for the best SWR, you are counter-productive.
REALITY: all that is theory, but in most cases the difference between an
antenna in resonance or adjusted for minimum SWR is not detectable at the
other end of the QSO, and a low SWR does keep our transmitters happier;
especially the under-dimensioned low cost Japanese transceivers!
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Bob McGraw
- K4TAX
Sent: Monday, September 15, 2014 9:04 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Zepp
In review of the ARRL Handbook, Ed. 2014, Chapter 21.7, there are clearly
two distinct examples of Zepp antennas. One being 1/2 wavelength and end
fed with open wire line and the 2nd being a center fed 1/2 wavelength wire
fed with open wire line.
On page 21.7 we find: "Another variation, shown in Fig 21.12B, is the end
fed Zepp, named for it original application as an antenna deployed from
Zeppelin airships. The feed point impedance of a Zepp is quite high,
requiring open wire feedline and impedance matching techniques to deliver
power effectively."
The Extended Double Zepp is described as having an overall length of 1.28
wavelengths which equates to 0.64 wavelength on each side of the center
insulator and feedpoint. This antenna is reported to have ~3 dB of gain to
its broadside. The length of the wire antenna is calculated as: 984/f(MHz)
x 1.28 = length in feet. The feed point impedance is ~140 ohms using wire
elements.
I find it interesting to examine the dipole antenna. By definition it is a
wire length being equal to 1/2 the wavelength of the frequency. The name
comes from two terms "DI" meaning two and "POLE" meaning electrical terminal
having two distinct regions of electrical polarity. A dipole is resonant
when it is electrically 1/2 wavelength so that the current and voltage in
the same antenna are exactly 90 degrees our of phase. The length of a 1/2
wave wire dipole is calculated as: 468/f{MHz} = length in feet. The feed
point impedance of a 1/2 wavelength horizontal dipole varies with height
above ground. The feedpoint impedance will vary from a very low value, 20
to 45 ohms over average real earth, when the antenna is less than 0.1
wavelength above ground to near 100 ohms at ~0.34 wavelengths above the
ground. At heights greater than 0.34 wavelength the impedance will
typically be 75 ohms +/- 5 ohms. The nominal 75 ohm value will be found at
heights of 1/2, 3/4, and 1 wavelength above ground.
73
Bob, K4TAX
----- Original Message -----
From: "Marty Hartwell" <mhartwe@gmail.com>
To: "Ten Tec list" <tentec@contesting.com>
Sent: Monday, September 15, 2014 10:49 AM
Subject: [TenTec] Zepp
> Hi
>
> I was reading the posts concerning the Zeppelin antenna. I did a search on
> the web
> and got some picture drawings showing what I am familiar with. Also
> someone
> said it was a trailing wire antenna, which is one I used from a bomber
> type in
> the Navy, which is simply a spool of wire attached to an automatic
> matching device
> and the wire is a random length, to install it we just strung out what we
> estimated to
> be over a 100 feet, no real measurement done. In some cases we were able
> to get
> a type of wire that was reinforced but usually it was just plain old 14
> gage wire we
> were able to pick up at a hardware store or some such.
> Yes Zeppelins may have used a trailing wire antenna I know our Blimps
> during and after
> WW2 had them, but the Zepp used on the Zeppelins was supported in each end
> of the
> main straight part of the top and fed with open wire feeder, and the
> feeder was whatever
> it took to reach the tuner/matching device.
> To the persons saying it is a G5RV antenna, it is not, the G5RV radiator
> is of a specific
> length, and has a fixed length for the 75 Ohm feeder, if I remember right.
> I have used
> a Zepp antenna, but not a G5RV.
>
> Marty kd8bj
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|