Posted: 8/17/2026 2:07:44 PM EDT
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But he sure found out the hard way
That dreams don't always come true
That dreams don't always come true
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Seems about right. The transmission line mismatch can exacerbate (or in some cases, help) other mismatches in the system. Loss itself generally isn't an issue, as opposed to transmit SWR, or detuning a filter or something like that. Not uncommon to use 75 ohm stuff on HF if you're using a tuner anyway. On VHF (or HF) you can tune the feedline length to a 1/2 wavelength multiple to eliminate the mismatch in a single band (or even harmonic) system. |
This is...a clue - Pat_Rogers
I'm not adequately aluminumized for this thread. - gonzo_beyondo
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I'm not adequately aluminumized for this thread. - gonzo_beyondo
CO, MI, OR - Please lobby your legislators to end discrimination against non-resident CCW permit holders
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Grok's right. Besides, RG-11 is getting harder and harder to find. The feedline to my trap/fan dipole is about 50/50 RG-8 and RG-11 and I routinely work DX pumping 5 Watts CW into it. SWR loss calculator Keep in mind also that to increase or decrease your signal at the receiving end by 1 S-unit, you must raise/lower your transmitter power by a factor of four. |
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When I made my first HF antenna 11 years ago I didn't have much money and a bunch (a few hundred feet) of some of the highest quality rg6 made left over from sat internet installs. I looked up the 1:1.5 swr at the time and I was going to be running a tuner anyway so I went with it and it's worked fine best I could tell. Three years later I put up another antenna and went with the rg6 for it. Now the first antenna's wire got cut and I need to replace it. I STILL have lot of the rg6 left and was more curious about exactly how much I would lose by using it. I can use the rg6 or spend around a $100 on coax. The rg6 is going to get used. 
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But he sure found out the hard way
That dreams don't always come true
That dreams don't always come true
| The only real problem I had was I couldn't find PL259 connectors the right size for it and still can't. I ended up getting the PL259 connector inserts for lmr400 (I think that's what I used) and putting a couple layers of heat shrink tubing on the rg6 to make it a snug fit in the insert. I still have some inserts left. |
But he sure found out the hard way
That dreams don't always come true
That dreams don't always come true
|
You can get connectors that fit RG6 now pretty readily: examples: https://www.ebay.com/itm/205000885367 https://www.ebay.com/itm/193254664075 RG8/LMR400 connectors should fit RG11, and RG8x/LMR240 connectors should fit RG59 |
This is...a clue - Pat_Rogers
I'm not adequately aluminumized for this thread. - gonzo_beyondo
CO, MI, OR - Please lobby your legislators to end discrimination against non-resident CCW permit holders
I'm not adequately aluminumized for this thread. - gonzo_beyondo
CO, MI, OR - Please lobby your legislators to end discrimination against non-resident CCW permit holders
| Just last week I was just using 75 ohm RG174 ( IIRC ) with BNC connectors between my IC-705 and Tuner connected to an EFHW. Worked just fine. I looked at the cable closely when I could not get the SWR below 1.5 and realized it was 75 ohm not 50 ohm cable. |
Mach
Nobody is coming to save us.
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Nobody is coming to save us.
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Originally Posted By d4xycrq: Only thing I've wondered is; do you trim the antenna for resonance at the feed point, or do you put your 75 ohm coax in the mix and then trim? I have several spools of 75 ohm coax. However if you are tuning the wire to minimum SWR, then you could trim the wire to out of resonance and have a 75 ohm feed point so that the only impedance mismatch is at the connection of the transmitter to the cable instead of that connection AND at the feed point of the antenna. If the cable is between the tuner and the feed point, the tuner will compensate and tune the cable antenna system as a whole.. If your wire is a true dipole with horizontal wires, the feed point at resonance will be 75 ohms, if you make it into an inverted Vee it will be at or closer to 50 ohms, so the reality is I don't think it really matters. Even if it makes a difference, the difference is minuscule compared to everything else. It is way more important to get the wire up as high as possible, at least above 1/2 WL to minimize ground losses which can be significant. If you have a 75 ohm cable connected to the transmitter to what ever, you will always have a minimum SWR of 1.5 because the finals are 50 ohms and the cable is 75 and you can't fix that and you will get reflected power right there all the time assuming it is solid state. Tube finals you will actually tune to a 75 ohm output impedance I think the key is to worry about the big stuff in efficiency of the system and IMO this is the small stuff. But like I said before, I could be wrong as I typically take the engineering approach of 'good enough' because everything is an estimate anyway and even the meters being used have tolerances and do not show the nitty gritty minute reality. Nobody typically accounts for the insertion loss of connectors and those are typically about 0.1 db per connector. So if the junction is loss from 75 to 50 ohm is 0.18 db, then having 2 connectors already exceeds that loss. Sure it all adds up, but it still adds up to no appreciable or noticeable difference. |
Mach
Nobody is coming to save us.
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Nobody is coming to save us.
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@d4xycrq How long a length of cable? If the calculator I'm using is anything close, RG-6A has a matched loss of 1.073 dB / 100 feet on 20M (14.0 mHz). This kind of loss is not really noticable on the HF bands. This presumes the antenna is a good matchof course. 73, Rob |
Sic semper tyrannis!
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@d4xycrq Sounds good, I may be mis - remembering something; but, I believe if the load is 50 Ohms, the transmitter will see 50 Ohms so to speak. Thus, the surge impedance, won't have much effect. I am pretty sure the load would have to be 50 Ohms resistive aka 50 Ohms + J0 with no reactance. That's what the theory says. 73, Rob |
Sic semper tyrannis!
| I still have a burried run of rg-6qs for HF. Works great. Tuned the antenna to mismatch so i would end up with offsetting mismatches, resulting in a functional antenna system w/o a tuner needed. Got a 250ft spool for $7. F connector kit for $12 at walmart, and f to pl259 adapters off amazon. Some coax seal tape and all is well. |
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Originally Posted By David0858: https://www.ar15.com/media/mediaFiles/14727/57525_jpg-3812899.JPG It's "RightWrong"  Please note this date and time and forum for historical purposes of the birth of the word "RightWrong" What grock meant to say was if the source impedance is 50 ohms. What the op asked is with his ham Radio setup, which is a much lower source impedance than 50 ohms. In a modern ham setup, there is no penalty to use 75 ohm coax. The "tuner" in the transmitter will adjust for the slight difference in impedance and output rated power. On the antenna end, simply adjust your antennas for 75 ohms !!! Random antennas will "match" 75 ohm closer than 50 Ohms exactly 1/2 the time. The famous EME station W5UN used 75 ohm coax and 75 ohm Yagi-Uda antennas. Purists will recalibrate their VSWR meters for 75 ohms (a few button pushes with today's vector network analyzers) Rege P.S. it's impractical to adjust a feedline to "multiples of a half wavelength on vhf or uhf", cable stretch and temperature changes are too great. Serious vhf and uhf stations use 75 ohm coax and 75 ohm antennas, most any other matching scheme has more loss. Rege One other thing, RightWrong on all things being equal with the coax. 75 ohm coax has lower loss per unit of material needed to make the coax. In other words, take a pound of copper and make 2 lengths of coax, 50 and 75, and the 75 will have lower loss per foot. https://www.electronics-notes.com/articles/antennas-propagation/rf-feeders-transmission-lines/50ohm-vs-75ohm-coax-cable-reasons-advantages-disadvantages.php https://www.microwaves101.com/encyclopedias/why-fifty-ohms Rege |
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I thought I remember reading a while back that 50ohm was a balance of line loss and power handling. 75ohm has less loss, but can't handle as much power. High power installs might use 33ohm coax (probably not anymore, not sure how old the article was) because it can handle higher power. I don't remember the power limits for various impedance coax, tho, or if it was even mentioned in the article. |
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Power handling is dependent on overall diameter and the type of dielectric used as the insulator. 75 ohm 1/2 inch catv "hardline" is available inexpensively, sometimes free. Here is a method to attach a pl259: https://www.w9xt.com/page_radio_gadgets_hardline_connectors.html Vhf/UHF stations that do not want to tune their antennas to 75 ohms can make a 61.2 ohm quarter wave matching section out of "hobby brass" tubing. https://www.repeater-builder.com/projects/matching-stubs.html Here is a PDF of loss for a 75 ohm 1/2 cable, power handling is tough to get manufacturer specs (nobody uses this stuff at high power in catv Systems), but I have never heard of issues at amateur power (1500watt) levels. https://productresources.commscope.com/admin/ImageServer.php?ID=f29f9dc7d4ca6f5a673c940470cc0adca2eb516a5ddab3aea0ae47f0041c5012:228905@pjk349&class=Mamfile&rand=409d3d1f2186852cd270b3bd9de94e7b&omitPreview=true&downloadname=CS_5201803_external.pdf You can also find 3/4 inch diameter 75 ohm catv hardline, a internet search will provide similar "kinks" There is a little used technique to use 1/12 wavelengths of 50 and 75 ohm coax to allow using 75 ohm coax in a 50 ohm system, mostly it's not worth bothering with. https://www.microwaves101.com/encyclopedias/twelfth-wave-transformer Rege |