W2DU Walt! SWR question...

[kc6mcw]

Walt,

   After countless experiments with multiband antenna systems, I have also finally discovered that running a high swr on coax due to the non resonate high feedpoint impeadance is very ineffecient. I now use a 200' wire center fed with 600 ohm open wire feeder and a link coupled balanced tuner and it works very well at 160,80,40,20.

   During many conversations with friends on the air, I am attempting to change thier way of thinking on the low swr issue. The common misconception they are having is if they see a "good" swr at the input of the coax, then it will still perform. They do understand it will have some losses if operated somewhere off of resonance but it is still an acceptable antenna to make contacts with.

   Example: A gentleman to whom I was speaking with, was using a resonate 80m V fed with 50 ohm coax into your W2DU balun. His signal was 20 over S9 using 100 watts. He then switched to his 160m V fed with 50 coax into your W2DU balun then retuned his tuner for a "good" swr for 80m use. With the same 100 watts, his signal dropped down to the noise level at around 6 s-units. I explained to him that it was an unacceptable antenna to use on 80m due to extremely high swr on the coax. BUT, he saw a "good" swr looking into the coax input when he switched his tuner to the straight thru or bypass position. Same gentleman also sees a "good" swr across many bands using his 160m loop fed with 50 ohm coax with no balun. He is using his MFJ analyser at the input of his 43' long coax and viewing the swr at no more than 5:1. I am having a difficult time explaining that the loop feedpoint impeadance will be different than the coax input impeadance due to the coax acting as a transformer. He says operating 80m on this 160m loop has the worse performance. But everywhere else does good including 10m. I explained the very low takeoff angle on 10m would be the reason for good 10m results but losses in the coax still exist.

   I dont think some people understand how much of a signal decrease thier station exhibits when they unknowingly operate a high swr on coax. They are happy if the tuner tunes it regardless of any losses. I was once that guy, until I noticed my coax was getting hot. Ham radio has never been the same for me after that day.

   Walt, thank you for taking the time in putting together one of the best books written on transmission line reflections.

Joe Townsley
KC6MCW
   
   
   
   

[W2DU]

Hi Joe,

Jean and I were just about ready to go out for dinner, but Rick got to me by phone, alerting me of your post.

Thank you, Joe, for the nice words about my book.

So here's my take on your situation. The operative word is IMPEDANCE. With a 200'-long dipole the terminal impedance is good for 160, but on 75 the impedance would be very high with a high level of reactance. If you were considering a dipole about 40 or 50 feet longer it would be resonant on 160, which would make it two half waves in phase on 78 or 80m. In this case the terminal impedance would be around 4000 to 5000 ohms, which means the mismatch to a 50-ohm coax would be around 100:1 at the termination, but with coax attenuation and that high a mismatch, the SWR reading at the input could be around 5:1. In this case most of the power from the tx would be lost to heat in the coax, and very little would be left to radiate from the antenna.

What I'm saying is that it's totally wrong to even try to center feed an antenna with coax at a frequency where the antenna is a full-wavelength long, which is nearly the case with a 200' dipole on 75m.

You are doing the right thing by using ladder line to feed the 200' dipole. It's true that the mismatch is still high, but not nearly as high as when fed with coax. However, that's not the point. Ladder line having a 600-ohm Zo has such low attenuation that it can handle whatever mismatch occurs at the antenna terminals, with negligible loss of power in heating the line. In addition, with a properly designed tuner to match the ladder line to the tx, the loss due to attenuation in the tuner is likely to be less than 0.5 dB.

So the gentleman who sees a 5:1 SWR at the input to the coax when feeding a 200' dipole on 75, and thinks his SWR is ok, simply lacks sufficient knowledge of transmission line theory and technique to realize that his SWR is NOT ok.

Depending on how far from you he's located, the fact that he switched to a vertical antenna could be the reason for some loss of received signal strength, but not to the degree resulting from the excessive mismatch terminating his coax.

Hope this helps, Joe,

Walt

[kc2ifr]

A high SWR on balanced feeders is not nearly as critical as it is on coass. Ive been thru this myself with the G5RV folks. I get a little sick of trying to explain this to them.
Bill

[K5UJ]

One quick bit of advice Joe:  Don't make the common mistake of equating a vswr null with "resonance."  An antenna is resonant on some frequency when its feedpoint impedence has only a resistive component i.e. there is no reactance.  But that measurement may be hundreds of ohms with no reactance.   That would be a huge mismatch with 50 ohm line and a high vswr.  But at that frequency the antenna would be resonant.  Which  means "resonance" doesn't really matter.  What's important is getting the feedpoint tuned so your unbalanced line has a nice 1:1 vswr on it so it has lowest line loss.  some mismatch is okay like using 50 ohm coax with a 75 or 36 ohm feedpoint.  With wild mismatches you either have to get the tuner to the feedpoint or tune the feedpoint in the shack, which is how I view balanced feedline when used with a balanced antenna--it is a little like extending the antenna feedpoint all the way to the tuner. 

73

Rob K5UJ 
 
p.s. you can go round and round on this with some hams--there is a K6 who swears up and down that he has "resonant" dipoles on every band and that an antenna has to be resonant in order to work.  He means of course that he has a low vswr but he cannot be made to understand the concept above.  I have given up with some of these guys.  If your friends don't get it...oh well, sometimes you have to move on.

[Steve - WB3HUZ]

I have given up with some of these guys

The best thing to do. You can walk away with the satisfaction that they will be piss weak and you won't.   

[K1JJ]

Yes, any length wire will take and radiate power if matched properly. If too short the losses will eat up any gain.  In fact, if we could use super conductors with near zero ohm resistance (.0001 ohms, etc) you could hold a fully functioning, efficient 75M rotary 3 el Yagi in your hand. (using heavy loading super conductor coils)  Just put it up at 130' and you could not tell the difference to a full-size one in pattern or efficiency. It would be about 0.1db more efficient than an aluminum full-size 140' Yagi...    Just like ripples from a stone thrown in a lake, RF pattern formation needs only a point source, not a 1/2 wavelength dipole, etc.

For the real whirl with wire: A ~near-resonant dipole length or shorter (around 1/2 wavelength or shorter) is the only way to guarantee a near perfect broadside figure-8  classical dipole pattern. Shorter, the pattern is OK but the impedance gets very low and tougher to match w/o loss, even with openwire when put to an extreme --  longer, and the lobes start to get sharper and finally split into many.


Multiple legs with openwire are really a great way to go.  I like the idea of 160 ft flat top to cover 160-40M and  22 ft flat top to cover 20-10M.  (40M still looks OK without breaking apart - and 160M is slightly less than 3/8)


A resonant dipole just happens to be a good match for 50-75 ohm coax - probably the reason why coax was developed with this impedance.   Openwire is perfect for multiband, curtain arrays, and higher impedance stuff.

Personally, I use coax and hardline for everything these days. I use openwire rarely for tuning a reflector from the ground or driving a pair of half-waves-in-phase or other high impedance projects.

T

[K5UJ]

I have given up with some of these guys

The best thing to do. You can walk away with the satisfaction that they will be piss weak and you won't.   

Thanks Steve, what happens is some of these guys cram 10 dipoles into a city lot, then they're all interacting with each other....

[flintstone mop]

There's nothing wrong at all with resonant antenna systems.You can bury the feedline and run it anywhere you want to go. Lower RF voltages, etc.

A disadvantage would be limited bandwidth (2:1 SWR for the solid state folks), especially on the lower bands.

Us ladder line folks have unique problems too: Ladder line needs more care for installation, the tuner has to deal with high RF currents and voltages, and on the theory-side, using a very long wire on (160M) on 20M or higher creates unwanted lobes and pattern distortion.

Fred

[ke7trp]

Its obvious to alot of us that open wire line and a balanced tuner is a great Multiband antenna over a Coax fed multiband antenna. 


 One thing that is not decided on is which one is the better antenna on the resonant band?   


Clark

[K1JJ]

Its obvious to alot of us that open wire line and a balanced tuner is a great Multiband antenna over a Coax fed multiband antenna. 
 One thing that is not decided on is which one is the better antenna on the resonant band?   
Clark

Clark,

On the bands below 30mhz, for single band use, there is virtually no difference in performance in operation.  The biggest AM signals on the air, like WA1QIX, K1KW, W2INR, etc, use hardline and coax. And, there's big signals with openwire. Both must be implemented well to work well.

For me, (or many thousands of corntesters running competitive stations) we run optimized antennas for a particular band, sometimes for a narrow band of frequencies. Each antenna has a specific job. For example, a fixed 3el  75M wire Yagi on Europe.  The application is so narrow, why bother with openwire? The reflector and director tuning is for a narrow bands of freqs, so why bother with openwire? Well-matched hardline run up to the tower and then transfered to coax at the feedpoint is within a few tenths of a db of openwire. Add in the tuner losses and it's a wash. There's no doubt hardline/coax is easier to use and maintain.  On VHF/UHF, when was the last time you saw anyone, amatur or commercial running openwire? That's cuz the loss is pretty close and the environmental stability is superior with hardline/coax.

However, for MULTIBAND antenna situations, or simply displaying a CLASS antenna act, openwire and a homebrew tuner wins hands-down, in my opinion.  Using coax for MULTIBAND antennas is simply burning up power needlessly in traps, loading gimmicks and coax. 

For COAX single band operation, a good method to extend bandwidth (and probably the only efficient way) is to run a FAN dipole. To cover 3.500-4.000 mhz:  Use TWO sets of legs - a 3.650 and 3.875 mhz set to keep the swr reasonably low across the band with minimal power loss.

T

[Steve - WB3HUZ]

Neither, since they are both resonant.

 One thing that is not decided on is which one is the better antenna on the resonant band?