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Can I use TV coax and a cheap TV balun to build a poor-man's OCFD? Marginal performance is acceptable if it saves money. If I could listen to 10% of transmitters within a 100 mile radius that would be acceptable. And as for the coax length, probably no more than 20 feet.

Just getting started and not a lot of cash. Want to do lots of listening on both my RTL-SDR and Pixie 2 40M QRP kit. (Probably would build two antennas, one for each radio.) I already have a 75-300 ohm TV balun and lots of CATV coax. Per this page when an OCFD is 20-25 feet above ground I should use a 4:1 balun, which matches the 75-300 ratio, if I am not mistaken. And 25 feet is about the max height I can get by slinging up in the trees around my house.

I am aware that using 75 ohm line on a 50 ohm Pixie will result in at least a 1.5:1 SWR, if not higher. And the RTL-SDR already has a 75 ohm input, so there's that. But I'm not going for excellent performance, just want to be able to listen to some of the transmissions in my region. Would be cool to pick up overseas as well, but that may be wishful thinking.

I may transmit, but that would not be a priority.

Oh and it wouldn't be strung up 24x7, just when I wanted to go listen then taken down the rest of the time. So I'm not concerned about lightning/rain/snow/wind.

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  • $\begingroup$ Have you looked up the difference between your particular coax and something better like rg-59? You will find that there is a lot of variation. Short runs may not be a problem, but if you know there will be loss using cheap coax then you know it shouldn't be used for long runs, eh? $\endgroup$ – SDsolar Aug 16 '17 at 21:35
  • $\begingroup$ Sorry but I have trouble quantifying your question... "better" than what, "short"er than what, and "loss"ier than what... But then I didn't give a quantity in my question, either. "Marginal" I suppose if I could listen to 10% of transmitters within 100 miles that would be acceptable. And as for the feed line, probably no more than 20 feet. Will update my question. $\endgroup$ – SlowBro Aug 16 '17 at 21:51
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    $\begingroup$ @SDsolar I have. CATV coax is low loss, especially if it's F-6 or RG-6, same loss as RG-213. $\endgroup$ – Mike Waters Aug 16 '17 at 23:47
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    $\begingroup$ Touche', @Mike. I was thinking about this 50 foot roll of flexible white stuff with F connectors on both ends that I have here in the workshop. Brand X. El-Cheapo. Probably only 50% braid. ;-) That's why I left wiggle room in my comment. $\endgroup$ – SDsolar Aug 17 '17 at 2:06
  • $\begingroup$ @SlowBro, if it is what you have on hand, then by all means give it a try. It might surprise you. If it is RG-6 it will be pretty stiff, and that is decent cable. That and 213 is what they use to connect your house to the utility pole. So rather than simply assume it isn't any good, and in the absence of the numbers, I would throw it up and see what it can pull in. I have the RTL-SDR and for longer runs have tried a variety of preamps, btw, and sometimes they help but other times they bring in as much noise as signal so aren't worth having in-line. Every connection adds loss, too. $\endgroup$ – SDsolar Aug 17 '17 at 2:10
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OK, let me try to answer this, but this answer may also be qualified as unqualified.

If you have a 50 Ohm receiver, and connect a perfectly (Z=R) 75 ohm antenna system, then your VSWR would be 1.5, and the "load mismatch attenuation" will be about 0.177dB. (with antenna system I include feedline)

I doubt that you would actually notice this.

However, you are using an RTL-SDR dongle with a source impedance of 75 ohm, and your feedline is 75 ohm, you use a proper 4:1 balun for a 300 ohm antenna, then you should be good to go. Even when you change over to a 50-Ohm receiver the loss due to mismatch is negligible.

The unknowns here is the suggested balun, and its behaviour when used with the OCFD. Unknown is also the quality and specifications of the "TV coax" you are planning of using.

As you are starting, and as you are only listening (no TX), I would actually fully agree with the answer given by Mike Waters unqualified or not.

Without knowing the very specifics of the TV-balun, and TV-coax, it would be hard for anyone to come up with a qualified or quantified answer for your particular question.

However, I did started off in this hobby as a SWL. And I have used unspecified components and material to build a range of antenna's.

  • rule 1: any antenna is better than no antenna
  • rule 2: any feedline is better than no feedline
  • rule 3: it is a hobby in which you can go mad... watch your wallet
  • rule 4: it is fun to experiment

For antennas I have used from rain gutters to water pipes, as feedline I have used from TV-coax to speaker-wire.

I have used a step-ladder as antenna...

Have I used TV-baluns ? -- yes I have, with various succes, pulled it down, swap in/out, try again

Have I used "some coax" ? -- yes I have, with various succes, sometimes it was just deaf... which means: replace.

Have I used a TV-balun with TV-coax and a OCFD ? -- no, I have no practical or qualified response to that...

As an SWL, work with the materials you have, work with what can be got cheaply, experiment, direct feed your OCFD, use the TV-balun... the succes rate is determined on what you hear, not what is calculated upfront. Then if you want to squeeze every single micro-dB out of your antenna system, then you can look at different materials/components, and budget for such.

And even when you calculate all this upfront, the moment you hang it outside in a tree or near a building, over real ground, the calculations may be off.

You do mention "may transmit", and just a word of caution here. If you really want to TX, then I would suggest to design/build an antenna for this with known components in order to avoid damage to your transmitter. This answer was written for RX not TX.

Have fun! It is a great hobby, and you will be surprised what you can receive with very little.

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  • $\begingroup$ A step-ladder. Now I've heard of everything. As for the quality of the coax, I need to go back and look at what I've got on hand. If I recall it's stiff, CATV excess. If the antenna meters at under 2:1 do you believe a Pixie transmitting at around 300mW could be damaged? $\endgroup$ – SlowBro Aug 17 '17 at 14:32
  • $\begingroup$ @SlowBro : that would make an excellent separate question. First thing to do is to check the documentation. (I believe it says <2.0... so 2:1 is theoretically to high). But with the unknown balun you are using, you do not know how that behaves when you TX, as it is designed for RX... so I would not take that risk if you don't want to take the hit of buying another Pixie. $\endgroup$ – Edwin van Mierlo Aug 17 '17 at 14:42
  • $\begingroup$ What would you need to know about the balun to better answer the question? It's similar to this, paid about a dollar on eBay some time back. amazon.com/300-Ohm-UHF-Matching-Transformer/dp/B0002ZPIOG $\endgroup$ – SlowBro Aug 17 '17 at 15:10
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TV coax typically has foam dielectric and very low loss. On receive it does not matter if you use 50, 60 72 or 75 ohm cable. With short cables, losses are also not important. The cheap TV balun could be problematic, particularly if you want to transmit. It is trivial to make a 1:2 transformer to get the 4:1 impedance transformation you need, but it is hard to get bandwidth larger than 10:1.

I suggest you go ahead without worrying so much about what is theoretically optimum. Start activity - then find what are the annoying limitations of your set-up. Once you know, (if you find any) you can put more interesting questions to the forum:-)

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Just Do It! :-)

You should build it with those materials you have. What have you got to lose? I believe it will exceed your expectations.

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    $\begingroup$ @SlowBro My answer was based on my experiences. $\endgroup$ – Mike Waters Aug 16 '17 at 23:44
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    $\begingroup$ Yes, but as it stands it is basically an unqualified opinion. Some further details would make this a better answer — such as an example of your experience. $\endgroup$ – Kevin Reid AG6YO Aug 16 '17 at 23:54
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    $\begingroup$ @KevinReidAG6YO Sorry you feel I'm unqualified. I don't have the time to say more than I have, I just wanted to help him. $\endgroup$ – Mike Waters Aug 16 '17 at 23:57
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    $\begingroup$ I didn't say you were unqualified: I said that your answer was, and I mean it in the sense of lacking detail or conditions, not lacking credentials. $\endgroup$ – Kevin Reid AG6YO Aug 17 '17 at 0:01
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    $\begingroup$ That's the way I read @Kevin's comments: the lack of numbers makes it an opinion. We all know already that you have a lot of experience, Mike. I doubt Kevin would have meant it any other way. Meanwhile, I like your answer here. "Just do it" is the answer I would accept if it were me asking this question. While I wouldn't do such a thing in a commercial station, this is ham radio, after all. $\endgroup$ – SDsolar Aug 17 '17 at 2:19
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I agree with the previous answers for your SDR receiver. But for the Pixie, I would do this (and you said that you wanted a separate antenna for the Pixie)
1. Simple end-fed wire 66 feet long for the Pixie antenna. This is a half wavelength at 7 MHz. Attach the wire to the antenna terminal of the Pixie. String it as high as possible.
2. counterpoise of 33 feet. Attach to the ground of the Pixie. this can lay on the floor or ground.
With a similar setup, I "worked" (contacted) a station 40 miles away - antenna and Pixie. They really work. Have fun!
W3TTT

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  • $\begingroup$ Yes since reading about this some more I've decided to build an end-fed half-wave. Grateful for your field report! But will build from scratch an MEF-1 tuner with LED SWR indicator. Found schematics for both and I already had most of the components. Simple build, cheap, and better impedance matching than straight end fed into the Pixie. $\endgroup$ – SlowBro Aug 24 '17 at 14:49
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A direct answer to your question: Absolutely!

There is nothing wrong with hobbling together an antenna out of the materials you have on hand. And, I think you will find that your proposed approach performs well for SWL with an RTL-SDR.

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  • $\begingroup$ No worries at all. I'm not here for the points anyway. :) $\endgroup$ – user2104506 Aug 17 '17 at 14:54
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FIRST; THE CHEAP T.V. BALUN I PRESUME 300 / 75 OHM SECOND, IF U WANT TO USE T.V. COAX AND ARE RUNNING QRP IT SHOULD WORK. BUILD A BALUN. LOOK 'EM UP ONLINE. THIRD; I BUILT AN OCFD FROM LEW MCCOY'S "LEW MCCOY ON ANTENNAS" IT IS FED W/ DUAL 75 OHM COAX. YOU CAN SEE THAT ANTENNA HERE [http://www.hamuniverse.com/kf9focfdipole.html]. I USED RG-6 BECAUSE I GOT 200' OF IT - FREE - QUAD SHIELD CABLE T.V. STUFF. YOU CAN USE '59, '58, '8X AT DIFFERENT V.F. THE FEEDLINE IS 130% OF A QUARTER WAVE AT THE LOWEST DESIRED QRG. I USED 3.9 MHZ AS A DESIGN QRG. THAT GIVES [234 / 3.9 = 60 X 1.3 = 78'. I HAVE HEARD FROM GUYS IN BRAZIL, INDONESIA, AND MANY OTHER LOCALES ON HOW HAPPY THEY WERE W/ THE LOW NOISE ON THIS ANTENNA. FED OFF CENTER 57.5' IN, USING DUAL SHIELD [IN THE FORM LIKE TWINAX], HAS HAD EXCELLENT RESULTS. '73 RICH - KF9F

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  • $\begingroup$ Why all caps??? $\endgroup$ – Timtech Nov 8 '17 at 1:00
  • $\begingroup$ @Timtech This is why. He is going blind. $\endgroup$ – Mike Waters Nov 8 '17 at 2:53
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    $\begingroup$ @MikeWaters Thanks, wasn't aware. My apologies to you, Rich. $\endgroup$ – Timtech Nov 8 '17 at 5:38

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