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1-6 of 6 messages
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75:50 ohm UNUN
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by VE4AEK on May 5, 2008
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I would like to build a 75:50 broadband transformer (UNUN) to match 75 ohm coax to 50 ohm coax for use on HF. I found plans for such a UNUN in an old ARRL handbook (1989) but the design is based on a FT-200-61 toroid core which I have been unable to find. I have checked a number of websites (Amidon, Palomar, etc.) without success.
Does anyone know of a source for this core or know of another design using available components?
Thanks,
Bruce - VE4AEK
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RE: 75:50 ohm UNUN
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by N3OX on May 5, 2008
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Bruce,
Seems like an FT-240-61 is pretty widely available and I think it would work fine.
Is this a high power application? I accidentally recieved some B-002-61 binocular cores (sort of 1.2 inch square, .6 inch thick binocular cores) from CWS Bytemark
http://cwsbytemark.com/prices/wideband.php
and I used them to build a couple of 75:50 ohm ununs .. think just three or four quintifilar passes through each hole, and they seem to work OK from 160m thru 10m @ 100W level. I used the wire from inside CAT-5 cable. I built two of them and wired them back to back and measured something not more than 0.13dB loss apiece, and no noticeable heating maybe 30 second keydown.
Now, I didn't make a very good loss measurement; just a wattmeter, from memory got at least 94 watts out of the back to back pair for 100W in, and I don't remember if I tested them across the whole frequency spectrum.
Of course, at 2kW they'd each dissipate nearly 60W and soon fail.
I don't necessarily recommend the binocular cores over the toroidal one, but it did make for a very nice, compact transformer that fits inside a BUD minibox. I think the moral of the story is that it's actually not all that critical what core and wire you use for these, at least at low power ;-)
73,
Dan
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RE: 75:50 ohm UNUN
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by WA7NCL on May 6, 2008
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Why bother, the mismatch is 1.5 : 1. Most transceivers consider this an acceptable match and the coax losses are not increased significantly.
For a single band you can also match it using a piece of 50 ohm line cut to the right length. Google Regier match or Branham match.
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RE: 75:50 ohm UNUN
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by N3OX on May 6, 2008
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"Why bother, the mismatch is 1.5 : 1. "
Yeah, from the antenna to the line... which sets the loss, but the transceiver could see something higher.
An odd multiple of a quarter wavelength of RG-6 will transform 50+j0 up to about 110+j0 and some transceivers will balk at that. It gets even worse if the load is lower impedance than 50 ohms. 35 ohms, 1.4:1 SWR on 50 ohm line, gets transformed up to around 150 ohms by an odd quarter wave length of 75 ohm line, and now the rig sees 3:1 and shuts down the output.
Changing line length will fix that of course, but if you start talking 40m or 80m, an extra quarter wavelength of line is a lot to have lying around.
So I think matching is appropriate here.
"Google Regier match or Branham match."
That doesn't work... but I think that's the same as a "twelfth wave transformer"
And they'll actually work well over a few bands. My 20m/17m moxon uses such a match cut for 16MHz to transition over to RG-6/QS type coax.
There's another series section matching setup (maybe associated with one of those names) that breaks the transformation into shorter chunks for even wider bandwidth and I used one of those to match 75:50 ohms from a magloop that tuned 5-21MHz and that seemed to work fine. Those series-section matchers are quite wide, really... but probably won't cover 160m-10m like a ferrite core would. You also end up with a lot of connections to waterproof or a big coil of coax inside too.
Great technique, though, and certainly something to have in your bag of tricks.
Dan
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RE: 75:50 ohm UNUN
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by VE4AEK on May 6, 2008
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Thanks, Dan and Bill, for your comments and suggestions. I should have described what I am trying to do.
I am putting together a new station for HF after an absence from ham radio of many years. Initially, I plan to run 100W to an N4GG Array (see ARRL website) for 40 and 20 meters. The N4GG Array is a fan arrangement of 3/2-wavelength dipoles with the outer 1/4-wavelength of each dipole element hanging vertically. The feed-point impedance is 73 ohms, hence the 75-ohm feedline. I will be using a homebrew 1:1 balun for 75-ohms (G3PTN design) at the feedpoint. The feedline will be around 125 ft and I am looking at using Belden 1694A RG-6 cable with upgraded (Canare) F-connectors.
I was hoping to minimize my losses by using an efficient homebrew UNUN. (I have to change from the F-connectors to UHF connectors at some point anyway, and I thought this might be done at the UNUN.) Of course, I don't want to introduce any significant losses with the UNUN since in that case I might have been better off living with the small 75:50-ohm mismatch!
Bruce - VE4AEK
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RE: 75:50 ohm UNUN
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by N3OX on May 7, 2008
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If you're feeding a 75 ohm array with 75 ohm cable you're not going to run into any excess mismatch loss (that comes from antenna/transmission line mismatch) and you're not going to run into the SWR problems that I wrote about.
A matched 75 ohm line of any length will show 75 ohms...
Your rig will deliver full power into that... so I don't know that you need to do anything at all.
But, a UNUN will be nearly lossless so if you like to have your SWR meter stay down on the left peg, well, you can sure add it, and I think you won't have a problem substituting a slightly larger core.
I can't think of any good argument why you'd NEED to match the rig to the line in this case. All of my rigs are very happy to output power into 1.5:1 SWR.
Matching the line to the antenna is much more important so you don't get up to 100 or more ohms with a bad line length choice... and that's already covered by your choice of antenna and feedline.
73,
Dan
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