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Author Topic: Radio Shack 300 ohm twin lead and end fed zepp  (Read 2241 times)
N8TI
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Posts: 115




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« on: January 25, 2012, 09:14:15 AM »

I thought it might interest some of you who were considering building an end fed zepp to hear about my experience with Radio Shack 300 ohm TV line.

I wanted to put a station in my bedroom closet.  I have a bunch of trees so I decided to put up an end fed zepp that could be used on multiple bands.  I concluded that I would use 50 feet of balanced line and about 100 feet or more of wire. As it ended up, I really don't know how long the wire is exactly, but it is probably between 100 feet and 130 feet.

I have ladder line but it is pretty big and heavy, so I looked at the Radio Shack TV line. This stuff has 20 gauge wires and the wires are encased in some form of hard foam with a brown plastic cover. It really is pretty rugged, as well as light, stuff. I had a hard time cutting into the foam.

Anyway, I used 50 feet of the line, connected it to the wire, hauled the wire up into the trees about 20 or so feet and ran the twin lead into the bedroom window.  The LDG antenna tuner tunes from 40 meters up without a problem.  I have not tried 80 meters.

To go into the tuner, I just used and alligator clip and a banana plug to get it to the center of the SO-239 on the back of the tuner and alligator clipped the other side to the ground of the antenna tuner.

The antenna itself works fine.  I won't know for sure how twisting the wire through the trees actually affects the pattern until I used it for a while, but for normal CW it seems to play well on all of the bands, with the tuner, of course. The SWR on 40 meters for example was like 9:1, and the other bands were all over the board. I definitely need a tuner, but I would have been shocked if I had not, given that the antenna was not supposed to be resonant.

For $19.95 plus tax at Radio Shack, 100 feet of the twin line seems fair. The outer covering over the foam which covers the wires is pretty thick.  I think that it will last every bit of ten years just looking at it. It is pretty light stuff, so there is no need to support it. And, it really is pretty easy to untangle, as I tend to tangle all my wire up while I install an antenna, and this time was no exception. I think it is a pretty cheap way to experiment. I am only running 125 watts and there does not seem to be any problem with it handling that. I imagine that it could handle much more than that.

Joe
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W5DXP
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Posts: 3574


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« Reply #1 on: January 25, 2012, 01:35:51 PM »

I thought it might interest some of you who were considering building an end fed zepp to hear about my experience with Radio Shack 300 ohm TV line.

To be entirely technically correct, an end-fed Zepp is 1/2WL long fed with (historically) 1/4WL of open-wire feedline. Thus a Zepp is technically a single-band antenna. When used on other bands, it becomes a random length end-fed wire. If you had a real 40m Zepp, your tuner probably would not load it on 20m.

Random length end-fed wires are notorious for common-mode problems. Have you measured your common mode currents?
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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Posts: 13231




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« Reply #2 on: January 25, 2012, 02:36:47 PM »

[That's what happens when I get interrupted in the middle of writing a reply -
someone else beats me to it.]


A lot of people seem to think that the Zepp feed works with random wire lengths.
Some commercial designs are promulgating this myth.  It really works best when
the wire is a multiple of 1/2 wavelength, and otherwise is just a random length
wire (including the feedline, which is simply part of the antenna wire in that case.)

Not that a Zepp feed is ideal in the first place, of course.

If your wire is close to 130 feet it should be working pretty much as intended
on 80 / 40 / 20 / 15 and 10m.  A bit longer might be better, but the shorter
wire is better for 75m.  Down around 100 feet will greatly change the current
distribution and (on most bands) the feedline will radiate nearly as much as
the antenna itself.


But back to the twinlead.  I've used standard commercial twinlead many times -
even the clear indoor type, which isn't as rugged as the stuff you were using.
It easily handles 100W as long as the load impedance is high enough.
Trying to feed a short doublet (such as a 40m dipole used on 80m) requires
high currents, in which case you'll be better off using a feedline with larger
diameter conductors.

The most common failure mode for twinlead is breakage at the feedpoint due
to flexing.  (This is true for ladder line as well.)  You need good strain relief
to the plastic insulation, not just the wires themselves, as the insulation
provides most of the mechanical strength.  Running the twinlead through a
couple slots in the feedpoint insulator will help.  I usually wrap the twinlead
around the insulator and secure the end to the standing part by wrapping
it with string.  If you are feeding directly into a wire without an additional
insulator, put a hole through the center of the twinlead and tie the wire
securely to it before connecting the ends.

One common problem with twinlead is that the losses go up when the line
is wet.  Waxing the outside (with liquid car wax) to encourage the water to
run off rather than standing or soaking in.

50' of twinlead should put you pretty close to 1/4 wave resonance on 80m,
and a half wave on 40m.  That means the impedance will be high on 40m when
the antenna is a full wavelength long, but it may be pretty close to 50 ohms
on 80m.  I used a balanced link-coupled tuner usually with my Zepp antennas,
or an outboard balun could be used with a coax tuner.  Some sort of balanced
feed is recommended if the purpose of the feedline is to reduce the radiation
from that part of the wire.
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N8TI
Member

Posts: 115




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« Reply #3 on: January 26, 2012, 10:30:12 AM »

I have not measured currents or really made any other measurements except on an Autek antenna analyzer (which showed 9:1 on a band or two, but where I was using it, it was out of range).

I do understand that an end fed zepp has a 1/4 wave feedline to a 1/2 wave wire, and I probably should not have described the antenna as a zepp.  Probably should have said a "zepp-like" antenna.

I have cats and don't have a place for a counterpoise that would ensure that they did not step on it while I was transmitting. By using the twinline, I thought that I could balance off the antenna by using one of the wires in the twinline instead of the counterpoise. It is certain, however, that given that the twinline is not 1/4 wave, the twinline would have to radiate, or at least I think so.

 The alternative was to work it against a cold water pipe, which I can make sure is a good ground by making sure there is a good connection all the way out to my well.

The layout of my trees did not welcome a centerfed antenna.  One of the nice things about a random wire is that you can throw it up in the tree without much fuss, especially in the middle of winter.

Of course, I have no idea of the pattern or how efficient the antenna is. But it seems to work OK, although, as is typical with a long wire, it does seem noisy, but it could be the solar storms we have been having that are causing the noise.  I have not compared it to my other antennas.

The proof will be how well it does over the next few weeks and months.  Perhaps I will get someone to compare how they hear it compared to my other antennas at the same time.

Joe
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WX7G
Member

Posts: 6028




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« Reply #4 on: January 26, 2012, 11:52:41 AM »

The twin lead you are using will work fine and I've used it and the clear Radio Shack line many times.

When dry the heavy #14 AWG ladder line has about half the loss of the #20 AWG twin lead. When wet the loss can increase significantly, especially on the closer spaced, fully plastic coated twin lead.

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N8TI
Member

Posts: 115




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« Reply #5 on: January 27, 2012, 08:23:11 AM »

FYI:

In Joe Carr's book, Practical Antenna Handbook, second edition, in the section on "Long Wire Antennas," he shows an antenna with ladderline feedline going up to a greater than two wave horizontal long wire, much like the configuration of an end fed zepp. He shows the ladderline terminating in the shack at an antenna tuner.  He talks about how the longer the horizontal wire is, the more gain there is over a dipole. I could not find where he give any direction about the length of the ladderline feedline. It might be that if the horizontal wire was so long, any radiation from the feedline would be minimal. He did not say that the horizontal wire would have to be resonant, only that it be greater than two wavelengths long. The tuner, however, was definitely a necessary part of this antenna.

He did not say that the antenna was a "multiple band" antenna. He just went on about how the longer you made the horizontal wire, the more directionality there was to the antenna.  He mentioned around that particular part about how his friend had a 1300 foot long wire and how it radiated in a way that was good for DX, apparently taking off at a low angle.

That part of the book is where I got the idea of my antenna as the wire part of my antenna is mostly horizontal, although my feedline is not.  In his book, Mr. Carr talked about how he felt that using a counterpoise of .05 of a wave length was all that was absolutely necessary for a long wire. I got the impression that the unterminated side of the feedline in the long wire antenna he described was a substitute for a counterpoise, although how the tuner was to cause the feedline not to radiate, I don't know.  Perhaps with the wire being longer than two wavelengths, which was a definite requirement for this particular setup, any radiation from the feedline would not make much of a difference.

His book is cool in the way he goes into showing how two long wires can get you gain.  Throughout his discussion of long wire antennas, he never seems to concede that they are not as good or don't have the gain or performance of a dipole.  He seems to always comeback to the fact that you definitely need a ground, either earth or radials.  In fact, he promoted the use of the MFJ artificial ground when an otherwise convenient ground was not available.

Perhaps his attitude was shown in light of the title of his book, "Practical Antenna Handbook." 

My lot line is 1300 feet long, so I am going to try that long, long wire Mr. Carr's friend had.  My lot line runs due west from the house.  I can probably get a wire of about 1100 feet or so at least out there. His friend had to terminate the end of the wire because of the noise.

The antenna I put up with the 50 feet of twinline and 100 feet or so of wire, seems to be doing well. I've been using only 30 watts and having QSO's around the U.S. lower forty-eight on thirty and forty meters CW.   I have not listened for DX yet.   
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KB4QAA
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Posts: 2367




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« Reply #6 on: January 27, 2012, 10:17:00 AM »

The ARRL Antenna Manual, 20th ed., says the EFZ can be used as a multiband antenna, with the wire length given as 1/2 L at the lowest frequency when used with an antenna tuner.

If unable to reach the required length the two legs of the feedline may be shorted at the radio and tuned as a random wire against ground.

Chapter 13 is dedicated to long wire antennas.  At 1/2L an end fed antenna has slightly more gain than a dipole, but it is split among 4 lobes vice two of a dipole.  By 3L long, the lobes have increased to 12 in both the horizontal and vertical directions.  Antenna alignment now become completely critical to any sort of performance.  By 5L long, there are 20 lobes.  It is hard to imagine that this sort of antenna has any useable advantages.

It is important to rely on antenna books that have documentation for performance claims. 

b.
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WB6BYU
Member

Posts: 13231




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« Reply #7 on: January 27, 2012, 12:48:49 PM »

There are lots of antenna designs out there that don't work as the author claims.
This is particularly true now that anyone can post anything in the internet.

Long wire antennas can be useful in some cases.  I've used wires from 120' to
240' with generally good success.  Longer ones have been disappointing.  We
tried a 600' long wire on Field Day and it was pretty dismal - the only good
thing about it was asking someone to go untie the far end when we were
packing up - it kept him out of the way for longer than he had expected,
not knowing how long the antenna actually was.

The main problem is that maximum radiation approaches the direction of
the wire, both in azimuth and elevation.  Yes, you get some gain over a dipole
in the major lobes, but you can quickly lose that if the wire is sloping, or at
a height where the ground reflection cancels radiation at that angle.

The behavior of the wire as a radiator is independent of the feedline, which is
why you didn't see any information about that.  It SHOULDN'T matter, as long
as the power is transferred to the antenna efficiently.  You can feed a long
wire with coax at a point of current maximum if you want and get the same
pattern.  The feedline length determines the losses and the impedance that the
tuner has to match, but not the radiation pattern of the antenna to which it
is attached.  (Except, of course, when the feedline is part of the antenna due
to common mode currents flowing on it.)

For the traditional HF ham bands a half wave wire on 80m with a Zepp feed was
usable on all the other bands as well (especially if it was cut for the CW end.)
But with the addition of the WARC bands this isn't always the case because
the wire might not be a multiple of 1/2 wave.

With essentially a random length of wire, there is significant current flowing
in the junction between the twinlead and the antenna wire, but not at the
open end of the other wire.  That means that the currents in the feedline are
unbalanced, and it will radiate:  perhaps not as much as a single wire, but
more than with a high impedance resonant antenna.  (There will always be
some unbalance and radiation with the Zepp feed anyway.)

I expect it to make lots of contacts, especially on 40m.  I've had good luck
with long wires running QRP.  But you likely have more RF radiating in the
shack than you would when the wire is a multiple of 1/2 wavelength, and
you may have some issues with tuner range or RF in the shack on some
bands.  The second wire does NOT replace the use of a quarter wave
counterpoise for reducing RF in the shack problems.
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