BOB, we were in sync until your last paragraph!
You said, move it up or down until you get lowest SWR.
If you meant for the purpose of a scientific experiment, OK.
Otherwise, the height should be chosen such that it maximizes the take-off
angle for where you are trying to work.
...On the low bands, 1/4 wavelength for NVIS (or lower with some additional
ground loss).
...Or 1/2 wavelength or higher for working long haul DX.
SWR is a [almost] meaningless factor in performance anyway as long as it is
close.
There is no significant advantage of improving SWR from 2:1 to 1.2:1, but
there is significant advantage in raising the height of an antenna (within
reason).
VF tidbits:
At my QRL we use two different types of wire:
... WIREMAN CQ-532, (AWG-18) Insulated, Stranded Copperweld, used for
elements in our wire beams
... WIREMAN CQ-534, (AWG-26) Insulated, Stranded Copperweld, used for
Top-Hat Wires with our verticals
In my tests while developing a 40m Asymmetrical Inverted-V antenna, I found
the difference between these two nearly identical types of wire to be 25cm
(exactly 1 ft.) when adjusting to resonance.
To my surprise, the thicker wire had to be 1 ft. longer than the thinner
wire in order to obtain resonance.
When working with aluminum tubing, it is the other way around.
Thicker tubing must be shorter, thinner tubing must be longer when adjusting
for resonance.
I have no idea why copper wire works different than aluminum tubing.
Lesson learned: 468/f is an approximation, not an end result.
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 11:11 PM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Zepp
Yes Rick. There are a couple of factors not clearly stated. In using the
468 number it does account for K factor which is the ratio of conductor
diameter to wavelength. However, the 468 does also presume one is using a
bare conductor. Using a wire that has insulation will change the length as
much as -5% from experience.
Agreed, hams typically do not have the luxury of adjusting antenna heights.
Thus if one has the antenna close to ground, less than 0.34 wavelength, and
moves it up their support, they will typically find the impedance goes up
and likely the SWR goes down. I suppose my point in all of this would be
ideally to first determine the resonant frequency using an analyzer or Grid
Dip Meter, adjust the length to put it at the preferred frequency, AND then
move the antenna up or down to get the SWR to the optimum value.
73
Bob, K4TAX
----- Original Message -----
From: "Rick - DJ0IP / NJ0IP" <Rick@DJ0IP.de>
To: "'Discussion of Ten-Tec Equipment'" <tentec@contesting.com>
Sent: Monday, September 15, 2014 3:39 PM
Subject: Re: [TenTec] Zepp
> 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
>
>
_______________________________________________
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
|