Coax puzzle

[sportygreg]

I was using cheap rg58 coax going to my Antron 99 and SWR about 1.3:1. Today replaced the cheap stuff with rg8x and now SWR at 1.6:1.Better coax,better connectors,why higher SWR?The only difference was using 17ft. of cable instead of 18ft.I had before.Still using 58 for jumpers-could there be a mismatch between cables?Should I try adjusting the tuning rings on the antenna?They are at the factory setting. :roll:

 

[BIONIC_TEXAN88]

i would try your rings

 

[trash]

The difference in return loss is due to standing waves in the coax. There readings that you get may not always be correct.
You should own a dummy load, connect that to the end of the coax under test and repeater the readings at several frequencies. The worst reading will be the most correct.

this is a good tutorial on testing transmission lines.
http://www.satvg.org/smf/index.php?topic=9.0

 

[W5LZ]

It probably is just the typical difference between impedances of coax. Not really enough to waste a lot of worry over. The 'quality' of the coax used has very little (if anything) to do with SWR. If that 1.6:1 bothers you, retune the antenna.
- 'Doc

 

[bob85]

you are comparing coaxial with 2 different velocity factors and your vswr is not perfect ( not 50ohm none reactive load ),

there should be a difference when changing similar lengths of different vf cables looking at a load other than the cables characteristic impedance,
not only what the meter says but the actual impedance as seen by the transmitter.

 

[W5LZ]

Actually, if the impedance matching is correct for the coax used, the coax's VF will make no difference no matter what the length is. Considering the accuracy of the measuring devices most people use, +/- 10% differences are very common and can be expected. That's assuming that the actual impedance of the feed line really is what it says it is, which ain't necessarily so anyway. I'd be more suspiscious if the SWR didn't change.
- 'Doc

 

[Jay in the Mojave]

Hello Sportygreg:

Two things maybe be happening here:

1 The newer Coax has less loss than the old RG58 Coax. So that means that if the new coax has less loss, then the antenna is seeing more power and returning more power. Just add a 1 dB Attunator Pad into the Coax and see the SWR decrease and the SWR bandwidth increase.

Some antenna installations that appear NOT have a SWR problem use long lengths of lossy coax.

2 Or the A99 Antenna is sending RF Energy down the outer shield
causing the coax length to show different SWR readings, by now being apart of the antenna. Add in a short length of coax like a coax jumper and see if the SWR changres.

Jay in the Mojave

I was using cheap rg58 coax going to my Antron 99 and SWR about 1.3:1. Today replaced the cheap stuff with rg8x and now SWR at 1.6:1.Better coax,better connectors,why higher SWR?The only difference was using 17ft. of cable instead of 18ft.I had before.Still using 58 for jumpers-could there be a mismatch between cables?Should I try adjusting the tuning rings on the antenna?They are at the factory setting. :roll:

 

[ken white]

you are comparing coaxial with 2 different velocity factors and your vswr is not perfect ( not 50ohm none reactive load ),

there should be a difference when changing similar lengths of different vf cables looking at a load other than the cables characteristic impedance,
not only what the meter says but the actual impedance as seen by the transmitter.

I agree with Bob85. Since the original installation VSWR was not 1:1, meaning the antenna feedpoint impedance is not purily 50 ohms resistive, then the coax will exhibit varying impedance properties as a function of its electrical wavelength. So when a different length and different velocity factor cable is used, VSWR will change and the antenna will need to be retuned if the new VSWR is not acceptable.

 

[sportygreg]

Thanks for the replies!I thought better cable would reduce SWR-I am going to retune the antenna and see what happens.I like SWR to be 1.3 or less although different meters give different readings!Radio meter says 1.3,Dosy says 1.6,and PDC5 says 1.4!Would a ferrite core near the antenna end of coax help?I have a few cores sitting around that would fit. :roll:

 

[Master Chief]

SWR is the relationship between the feedline (coax in this case) and the feedpoint (antenna), right where they meet (at the connector)!

If the SWR is correct, as pointed out previously, then the only effect the coax will have is its own internal loss.

Always use the best feedline you can afford! Changing to a better coax will not fix a problem that already exists, unless the problem WAS the coax!

The antenna's feedpoint is usually the problem!

 

[W5LZ]

Okay, here's what I take exception with.
If the input and output impedances (transmitter and antenna) are the same characteristic impedance as that of the coax, then the velocity factor of the coax plays no part in the resulting SWR. The only time when the feed line's velocity factor plays any part at all is when the coax is used for impedance matching or phasing/timing. In those cases, the difference in the characteristic impedance of the feed line and the antenna's input impedance, and it's length, is the 'variable' that you are playing with.
If I'm understanding the set up correctly, just a single feed line of whatever length, supposedly having an impedance of 50 ohms, then the most likely/obvious point of contention is the antenna's input impedance. So, since it can be varied, retune the antenna. Already suggested.
An SWR difference of 0.3 is probably the difference in the normal characteristic impedance between any two 'chunks' of coax. Not to mention the possible (and common) differences in readings between any two SWR meters!
- 'Doc

 

[Beetle]

Bottom line: tune the ANTENNA, not the feedline.

 

[ken white]

Okay, here's what I take exception with.
If the input and output impedances (transmitter and antenna) are the same characteristic impedance as that of the coax, then the velocity factor of the coax plays no part in the resulting SWR. The only time when the feed line's velocity factor plays any part at all is when the coax is used for impedance matching or phasing/timing. In those cases, the difference in the characteristic impedance of the feed line and the antenna's input impedance, and it's length, is the 'variable' that you are playing with.
If I'm understanding the set up correctly, just a single feed line of whatever length, supposedly having an impedance of 50 ohms, then the most likely/obvious point of contention is the antenna's input impedance. So, since it can be varied, retune the antenna. Already suggested.
An SWR difference of 0.3 is probably the difference in the normal characteristic impedance between any two 'chunks' of coax. Not to mention the possible (and common) differences in readings between any two SWR meters!
- 'Doc

Doc, I don't think anyone will argue that when all impedances are matched, such as when connected to a dummy load, the coax can be any length or velocity factor.

Typically what happens is that an antenna is installed at a location that makes the installation easy and not what is best for radiation. Then a fixed length of coax is installed between the transmitter and antenna that now has an unknown, or exact, feedpoint impedance.

Guess what, this piece of coax is now part of the tuned antenna system and it is almost certainly changing the reactance and resistance of the load, antenna and coax, that the transmitter sees.

So, the only way to take the coax out of the tuning process is if the antenna is tuned without coax, or with 1/2 wavelength multiples of coax. But even using 1/2 wavelength multiples will not be exact, because this value is only valid at one frequency, not an entire BW.

I guess my point is that the coax really can't be treated as an independent piece of the system unless the transmitters output impedance, coax, and antenna feedpoint impedance are all exactly the same, and the probability of this happing is a whole lot lower than the probability of this ideal set of conditions not happening.

So the coax's electrical length will nearly always have an affect on the VSWR.

 

[ken white]

Thanks for the replies!I thought better cable would reduce SWR-I am going to retune the antenna and see what happens.I like SWR to be 1.3 or less although different meters give different readings!Radio meter says 1.3,Dosy says 1.6,and PDC5 says 1.4!Would a ferrite core near the antenna end of coax help?I have a few cores sitting around that would fit. :roll:

Get yourself a good, calibrated, power meter and calculate VSWR using Forward (Pf) and Relected (Pr) Power. The equation is:

VSWR = (1 + (Pr/Pf)^0.5) / (1 - (Pr/Pf)^0.5)

This will almost always be more consistant, and more accurate, than the majority of SWR meters on the market.

 

[W5LZ]

Ah, but if the length of the coax always has an effect on the SWR of the system when operated away from it's design/measured frequency 'standard', why worry about it at all when away from that frequency? (Especially when the difference in SWR is so slight, and well within the measurement accuracy of almost any SWR meter.) The difference isn't in the velocity factor, but in the difference in impedances of system components, right? Same old story, re-tune the antenna.
- 'Doc


[And since the components of any antenna system will never be exactly what they are supposed to be, you typically find that a 1:1 SWR is more a miracle than fact. And, usually why anything around 1.5:1 is considered the same as 1:1.]