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1-8 of 8 messages
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Homebrew Windom 180
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by KD5NVC on October 4, 2001
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I built a windom 180 with the a few modifications. First the dipole section is 81 feet by 171 feet, the vertical is 22 feet of RG59/u. I have a 4:1 balun at the dipole feed point and a simple 1:1 RF choke at the end of the 22 foot section of feed line. There is 74 feet of RG 59/u from the 1:1 down to the radio.
The original design called for RG8x for the feed and vertical sections, I only had RG59/u.
This should have yeilded a 1.5 to 1 SWR on most bands but I measure near 8 to 1 on 10 thru 180 meters. Using a tuner I can get this down to 1.5 to 1 just fine.
My question is, since this design should have given me a low SWR, does the RG58/u feed and vertical section cause the higher SWR? If not, what should be my next step in getting the SWR down as low as possible?
Cheers and 73's
Glenn Breaux
KD5NVC
glenn.breaux@inteq.com
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RE: Homebrew Windom 180
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by WB6BYU on October 4, 2001
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The coax type should not make that much difference. When the
SWR is that high, it probably means there is something wrong with
the antenna - perhaps a short or open somewhere.
What bands are you using it on? Have you plotted the SWR
across each band to see if it trends up or down? Is the SWR high
on all bands, or just some?
If signals are fairly strong, but the SWR is high, it will often be due
to an open connection in the coax shield somewhere. If received
signals are weak, it could be an open center conductor or a short
circuit. Check the coax and balun with an ohmeter.
You can check your balun by putting a load resistor on the output
and checking the SWR on the input (using low power - so you
don't burn out the resistor.)
Given the length of coax you are using, I'd expect you would see
about an 8 : 1 SWR on just the coax with the balun disconnected!
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RE: Homebrew Windom 180
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by AC5E on October 4, 2001
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HI; First, check the feedline. The best way I have found is to hook the antenna end up to a SWR/Power meter and a dummy load. If you put 100 watts in you should get a substantial amount of power out.
The exact amount depends on the band and the length of coax - but if you are supposed to have 1 dB loss per 100 feet at the frequency of interest and you put 100 watts into 100 feet of cable you should get 79 watts out. And if you don't you know your coax is bad.
My next thought is that you really don't have either Loren Windom's original single wire fed OCF (off center fed) flattop, or the OCF dipole generally thought of as a "windom." If your feedline checks OK I think I would start by putting a current balun (see www.radioworks.com) at the feedpoint and connecting my feedline directly to the wire. That should work very well on 160 - and well enough on 75/80.
It will become increasingly "lobey," which is not all bad, on the higher bands and the mismatch at the feedpoint may be great enough to cause substantial recieved signal reradiation. Which can make you think your radio is broke on 12 and 10.
Hope this helps
73 Pete Allen AC5E
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RE: Homebrew Windom 180
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by KD5NVC on October 4, 2001
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Hello and thanks for the info. I checked the coax, which is OK and no shorts or bad connections.
I did notice that the 4:1 balun (with both legs of the dipole connected and without the coax) shows a dead short. I checked the balun alone prior to instilation and it had no continuity, is this normal?
The SWR is up and down from 10 to 160 meters but I have not logged the data as of yet.
The coax alone shows the normal loss for it's length and type, this should be OK.
It may be the balun is shorted?
Thanks
GB
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RE: Homebrew Windom 180
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by AC5E on October 4, 2001
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Hi again. Normally, any balun will measure a very low resistance between both the input and the output terminals. But your problem may well be the balun.
Actually, a "matching section" of some sort should not be necessary. I used an OCF dipole (windom) at the farm with considerable success. That one was cut to the usual 468 divided by frequency formula, and fed straight from the feedline, RG8X, through a current balun at 1/3rd of the way from one end.
I had to trim it slightly, a couple of feet as I remember, to get it to resonate at 1.840 - and the SWR was below about 3:1 on most bands.
Now, one thing that should be noted is that in theory, a dipole that has a 1:1 match to the feedline at its feedpoint will "intercept" half the energy that falls on it. And it will reradiate half of the energy it intercepts.
The higher the mismatch becomes the higher the amount of energy reradiated becomes. A really long dipole, long wire, etc., usually has a very high feedpoint impedance and a severe mismatch at the feedpoint. Therefore it reradiates most of the energy it intercepts.
So a half wave on 160 tends to be a very good antenna on 160, a pretty fair antenna on 75/80, and a passable one on 40. Which is why my 160 Meter windom is hanging on the shed wall at the farm, along with several other failed experiments.
But if it suits after you get it to work I have no reason to complain.
73 Pete Allen AC5E
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RE: Homebrew Windom 180
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by KD5NVC on October 4, 2001
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Hi Pete,
Thanks again for the info, I measured resistiance on the balun on each side of the output and each contact on the input. This was a dead short, at the time I put the system together, I measeured resistiance with the system all together and hooked to nothing. There was no conductivity on each side of the balun eye hooks nor was there any at the feed end of the coax.
Since there is now a short at the balun, I am assuming the balun is at fault. The coax is fine....
I may try this system without the balun and test results...you never know what may happen...
Thanks and 73's
Glenn Breaux
KD5NVC
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RE: Homebrew Windom 180
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by WB6BYU on October 5, 2001
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A typical 4 : 1 balun has all terminals connected to a common coil,
so it should look like a dead short (< 1 ohm) between any two
ports. So the measurements you are what would be expected.
(I can't expain the previous measurements, however.)
Here are some ways to test a balun:
1) put a load resistor on the output and measure the SWR on the
input. With a 4 : 1 balun, a pair of 100 ohm resistors in series will
do nicely - just keep the power down so you don't burn them out.
(But use non-inductive resistors, not the square power resistors
which are wire-wound inside.)
2) Load the transmitter into a dummy load through a coax "T"
fitting. Connect the balun to the open side of the "T" fitting using
a very short cable (or adaptor). Do not connect anything to the
output side of the balun. The SWR looking to the combination of
the dummy load and the balun should not change much from just
the dummy load.
3) If you have a second balun of the same type, connect two of
them back-to-back between your rig and a dummy load. If the
SWR is reasonable and neither balun gets hot at full output then
they should work fine at that power level.
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RE: Homebrew Windom 180
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by K4MZW on October 8, 2001
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I sold the popular Fritzel Windom dipoles from Germany for several years. This is perhaps the best windom design ever manufactured.
A true windom is fed at the 1/3 (66% of length) point. At this feed point the impedance will be 300 ohms requiring a 6:1 balun.
Your problem with high SWR could well be the matching balun is not right. Also a windom will not, should not, might not show acceptable resonance on 15 meters due to the dimensions.
Feedline length is sometimes very critical to prevent RF present on the feedline, it is also recommended that you not place any portio of the antenna or feedline near metal objects like fences, gutters, poles etc.
Read about windom antenna design in the handbook and I think you will find your answer.
Good luck,
K4MZW
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