Coax Length and SWR.

[gil]

Here is an interesting observation from a recent experience... It's one thing to know coax length does affect SWR, but seeing it so blatantly is enlightening. I had plugged in my RT-320 into my Windom through a 5m coax patch cable. The outside cable is 10m RG-174 from a window frame to the dual core Guanella BALUN. So a total of 15m from radio to antenna feed point. The meter barely moved on the 320, meaning a pretty high SWR. I switched to another 10m of RG-174 for a total of 20m this time, bingo! Three quarters deflection. On the 320 you want at least half.. I doubt you could damage the darn thing, but for efficiency you want a good match. I also once tried the indoors 10m of coax spooled onto a winder and got the same high SWR.

When using a tuner, it isn't so much an issue, especially if the tuner is at the antenna feed point. If not, you do lose some power. I always have two cables with me in case one breaks but will now carry different lengths to use with resonant antennas for better SWR.

Gil.

[cockpitbob]

If I recall correctly, what's going on here is the high loss RG-174 is reducing the power to the antenna, and then reducing what reflects back to the radio.  So the SWR at the antenna didn't change, but the reflected power measured at the radio is less with the added feedline loss.  Does this make sense?

[gil]

Hello Bob. Yes, but I do not think that is quite the case, not as much as I saw... It would be interesting to plug in a 5m thicker coax cable to compare of course. It is well known that coax length does change SWR.. I'll have to look it up, this is all I can remember right now.. RG-174 isn't very lossy on the lower bands, even if it is dismal on VHF/UHF. I believe there is some theory to be found on coax length.. I need to find it..

Gil

[KK0G]

Yes, changing feedline length may allow you to see a different SWR but the length doesn't actually effect SWR. It's difficult to explain, and honestly I barely understand the concept myself, but what you're seeing is really the effect of where the measurement is taken along the length of feedline and not the actual length of the feedline. Imagine a length of feedline with standing waves (the SW of SWR) superimposed onto it. If you move a meter along that length the measurement will change depending on if it's at a peak, trough, or where the wave is crossing the line. I read an article with images that explained it pretty well but I don't have a link to it right now.

[gil]

Correct yes.. I'll look for something online to post it here, but if anyone beats me to it, post it!

Gil

[KK0G]

I found an ARRL article that explains it though it's not the one I was referring to. The following paragraph from the article does a much better job of explaining it than I did in my previous post.




"In the case of a mismatched condition, something interesting happens along the transmission line. Before, with the matched antenna, the same voltage existed anywhere along the line. Now as you move along the distance of the line, the voltage will change. It now has peaks and valleys. The 33 percent reflection from the antenna alternately adds to and subtracts from the forward voltage wave. At some places on the cable the reflected voltage adds to 133 percent, and others it subtracts to 66 percent of the matched transmitter output. The voltage ratio is 133/66 or 2.0. That voltage ratio defines the SWR. The fact that the voltage along the line changes with position and is different from what the transmitter would produce is called a standing wave. Standing waves are only present when the line is mismatched."

[gil]

Interesting thanks! So I guess that for a matched antenna the length of coax would not matter... Am I thinking correctly here?

Gil

[cockpitbob]

Nice explanation KK0G!

[KK0G]

Interesting thanks! So I guess that for a matched antenna the length of coax would not matter... Am I thinking correctly here?

Gil

With a matched antenna there are no standing waves on the feed line. I'm in the camp of amateurs that think SWR is way overrated as a measure of antenna performance, remember a dummy load has a perfect SWR of just 1:1. Go back 30-40 years ago and an SWR meter was a pretty rare sight in most shacks, then during the C.B. craze they became cheap and plentiful and all of the sudden SWR was the benchmark of antenna performance whereas before few hams had even heard of SWR yet they made, tuned, and used antennas and made lots of contacts the world over with antennas that no doubt had extremely high SWR. The real issue with having an extremely high SWR is potential damage to transistor finals and high losses in coaxial feed lines. That's why my main big dipole at home is fed via a tuner/balun and twin lead feed line, there's been times I've had insanely high SWR on certain pfrequencies but the twin lead has very little loss under those conditions and as long as the tuner matches the whole shebang the finals in my transmitter are happy.

[gil]

I agree. Lots of transmitters today are also protected against high SWR. It surprised me to observe that a "random" end-fed wire is practically, if not quite, as good as a resonant half wave. You can load up anything with a good tuner.. Antenna size seems to be much more important. A good counterpoise for anything other than a half wave helps a lot too.. I happily transmit with a SWR up to 2.5:1. I'd rather not have tuner losses when using QRP. With the RT-320 I don't even care.. When using digital modes I'm a bit more careful, but CW or SSB, nah..

Gil

[Jim Boswell]

Most basic SWR meters measure VSWR. The voltage can change with the length of the coax or where you measure the swr, at the radio or at the antenna. I like the reflected power type of measurement like the older TenTec radios used. The byrd meters use this system also.

[Lamewolf]

About 3db of loss (half power) in 100 feet of RG174 at 14 MHz, so at that frequency you are looking at 100 watts in with about 50 watts to the antenna - not very efficient !  But ice may not form on your coax in winter when transmitting !  ;D

PS: That is in a matched condition, as swr rises so does loss !

[Lamewolf]

I agree. Lots of transmitters today are also protected against high SWR. It surprised me to observe that a "random" end-fed wire is practically, if not quite, as good as a resonant half wave. You can load up anything with a good tuner.. Antenna size seems to be much more important. A good counterpoise for anything other than a half wave helps a lot too.. I happily transmit with a SWR up to 2.5:1. I'd rather not have tuner losses when using QRP. With the RT-320 I don't even care.. When using digital modes I'm a bit more careful, but CW or SSB, nah..

Gil

Gil and gang,

Even though the radio may be protected does not exclude losses in the system.  The system being feed line, connectors, tuners, antenna etc.  All of these components can introduce losses, and so does a mismatch.  To figure efficiency, you have to take all of this into account.  For example, if you have 3db of loss in the feedline, you lose about half the power you put into it in the form of heat in the feed line, so again - 100 watts in, 50 watts effective radiated power (ERP).  Now couple that to an antenna that has 3db of gain which effectively doubles the power, and you are back to 100 watts ERP, but if the antenna is an isotropic radiator (no gain), your ERP is still 50 watts.  So even thought the radio is protected doesn't mean you are operating into an efficient load.  So with your RG174 coax, you could see a big improvement by using cheap RG58 coax which only has about 28% loss in 100' at 14 MHz compared to 50% in the RG174 !

[gil]

Certainly.. I like RG-174 but for short runs only and not above 20m. I do use RG-58 for more than 30ft. I have never used more than 66ft of coax. For VHF I use LMR-400. The reason I try to not use tuners is to avoid losses. The tuner indeed does nothing for efficiency, except when placed at the antenna, like on an end-fed, which avoids too much coax losses due to the mismatch..

Gil

[Lamewolf]

If you want a true swr reading on your antenna, then use an "ELECTRICAL HALF WAVE LENGTH" of coax which duplicates at the shack end what the impedance is at the load end.  By "electrical half wave" I mean a coax that is cut to length by factoring in the velocity factor of the given coax.  So if the velocity factor is 66%, you would do the math of 468/Freq in MHz = length in feet X .66 or 468/3.5 = 133.71 X .66 = 88.25 feet.  Once you get the antenna resonant at 3.5 MHz with that length of coax, it shouldn't matter what length of coax you use because it will be looking into a purely resistive load.  The reason different lengths of coax effect swr is because its not a purely resistive load and is effected by reactance.  Doing it this way is pretty much splitting hairs but it is more accurate !