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Strange question about SWR's

longhaireddwb

longhaireddwb

W9WDX Amateur Radio Club Member
Oct 8, 2008
555
30
38
Colorado Springs, CO
A guy at work is having a hell of a time with his radio. He has a Fire-stick antenna, a new RG8X coax and there hooked to a Cobra29 Classic.

He tells me he just can't get out very far. He sais you have to be next to him to hear him. Anyway, he said his SWR's were very high and he couldn't adjust his antenna any more to fix the problem. I told him to add a ground strap to the bottom of his antenna and run it to a good ground like the frame or something. Another guy told him to COIL HIS EXTRA COAX UP! He coiled the extra coax and now his SWR's are 1.1 across all channels! He still says he's not getting out very far so I don't think he really fixed anything.

WHY DID THIS MAKE A DIFFERENCE IN READINGS!!!

Are these correct readings or what?
 

The readings are correct in as much as the radio is now seeing a near perfect 50 ohm impedance, but RF is probably not being delivered to the antenna.

For example, a certain length of coax not connected to anything can show a flat SWR. A dummy load also shows a flat SWR. Neither of these radiate very much. Your buddy basically made a dummy load.

The problem is almost certainly a grounding problem. All the curled up coax is doing is fooling his radio (and keeping him from damaging it). Otherwise his radio is pretty useless.
 
longhaireddwb said:
A guy at work is having a hell of a time with his radio. He has a Fire-stick antenna, a new RG8X coax and there hooked to a Cobra29 Classic.

He tells me he just can't get out very far. He sais you have to be next to him to hear him. Anyway, he said his SWR's were very high and he couldn't adjust his antenna any more to fix the problem. I told him to add a ground strap to the bottom of his antenna and run it to a good ground like the frame or something. Another guy told him to COIL HIS EXTRA COAX UP! He coiled the extra coax and now his SWR's are 1.1 across all channels! He still says he's not getting out very far so I don't think he really fixed anything.

WHY DID THIS MAKE A DIFFERENCE IN READINGS!!!

Are these correct readings or what?
Click to expand...

It is most likely a grounding issue, as in he probably has the insulating washer for the antenna on the wrong side of the antenna mount thus grounding out the antenna.

The insulating shoulder washer should be on the top side between the antenna and the mounting bracket.
 
Thanks guys! I'll check the washer out myself and tell him what you all said. I had a feeling it was something like this. I learn more every time I post here. You all are the best!!!
 
Larryso

Its been a long time since I messed with an antenna. But I think that I remember that by trimming the length of the RG8X a little at a time you will get a match that will work for you. If you are using an SWR meter, put it at the antenna end and check there. I don't understand that by coiling the coax it made a difference in SWR, unless something else is in the picture.
Larry
 
You do NOT have to trim coax lenght to adjust SWR. I suspect the reason coiling the coax cable up made a differance is that the coax was acting as part of the antenna, most likely due to a poor ground, and had RF flowing on the shield. Coiling the cable up created inductance that choked off the RF preventing it from flowing down the cable and affecting the SWR readings on the meter. This is known as common mode currents.Check out all the grounds at each end of the cable and the radio itself.
 
Coiling coax does make for an RF choke. But that's not bad, it doesn't affect the signal -inside- the coax, it only 'chokes' RF on the outside of the coax which you don't want anyway.
- 'Doc
 
madvillebirdman said:
I have always heard that rolling up excess coax neatly causes an "RF" choke on the signal. I have always zig-zagged excess coax rather than winding it.
Good Luck. Check my post on "No Ground Antennas"
Click to expand...

WHY would someone coil up "excess" coax? Cut the cable to the correct length to reach, TUNE the Gosh-darned antenna to resonance and for-GET about all this coax nonsense!!!(y) If we are using coax to "tune" the antenna, then we don't NEED the antenna at ALL!!! Coiling up coax is another of these myths that become radio "gospel". We DO need the antenna, however, because it DOES more efficiently RADIATE the signal out into the air.

It is difficult to explain WHY we are having this problem on a forum. It involves several things that are difficult to explain. For a number of reasons, the antenna is not matching and it usually involves something called impedance. It can be caused by 1) insufficient ground , 2) improper mounting, 3) the antenna itself too short or long (NOT the coax). Is the antenna too CLOSE to the body of the vehicle? Is the body PLASTIC? Has the antenna itself been broken in some way (whacked by an overhead)?

If the antenna itself is OK, the mount is correct, the location is good and the system STILL will not "tune", then there are several solutions. I would highly recommend finding someone with an MFJ Antenna Analyzer that will read SWR and IMPEDANCE. By attempting to set the SWR while observing the IMPEDANCE, you will get an picture of what issues are present. If the antenna won't come into resonance and/or the meter won't read 50 OHMS, then you'll have a clue as to what's going on. Sometimes you can wind a small coil or use a small resistor(just a few experimental turns will do) and put it in series-series parallel at the feedpoint of the antenna. This MAY provide the correct impedance and present an acceptable SWR.
I know this is a complicated solution to what SHOULD be a simple problem. Normally, at 27 MHZ, such measures are not necessary. But, again, not knowing exactly HOW the system is installed.................................

Good luck with it.

CWM
 
C W Morse said:
WHY would someone coil up "excess" coax? Cut the cable to the correct length to reach, TUNE the Gosh-darned antenna to resonance and for-GET about all this coax nonsense!!!(y) If we are using coax to "tune" the antenna, then we don't NEED the antenna at ALL!!! Coiling up coax is another of these myths that become radio "gospel". We DO need the antenna, however, because it DOES more efficiently RADIATE the signal out into the air.

It is difficult to explain WHY we are having this problem on a forum. It involves several things that are difficult to explain. For a number of reasons, the antenna is not matching and it usually involves something called impedance. It can be caused by 1) insufficient ground , 2) improper mounting, 3) the antenna itself too short or long (NOT the coax). Is the antenna too CLOSE to the body of the vehicle? Is the body PLASTIC? Has the antenna itself been broken in some way (whacked by an overhead)?

If the antenna itself is OK, the mount is correct, the location is good and the system STILL will not "tune", then there are several solutions. I would highly recommend finding someone with an MFJ Antenna Analyzer that will read SWR and IMPEDANCE. By attempting to set the SWR while observing the IMPEDANCE, you will get an picture of what issues are present. If the antenna won't come into resonance and/or the meter won't read 50 OHMS, then you'll have a clue as to what's going on. Sometimes you can wind a small coil or use a small resistor(just a few experimental turns will do) and put it in series-series parallel at the feedpoint of the antenna. This MAY provide the correct impedance and present an acceptable SWR.
I know this is a complicated solution to what SHOULD be a simple problem. Normally, at 27 MHZ, such measures are not necessary. But, again, not knowing exactly HOW the system is installed.................................

Good luck with it.

CWM
Click to expand...
Is'nt that what wind the coax in a coil does.lol leave to theory to complicate the most basic of CB lore.
 
No. We are confusing a RADIATING coil with a device at the FEEDPOINT of the antenna that is used to transform a random impedance to 50 ohms. This device can be a little coil, a capacitor---even a resistor---- OR a toroid that is installed from the base of the radiating element (bottom) and taken to nearby ground. I don't want to get into a complex explanation of antenna theory, but what we are getting into is the mechanics of impedance. First of all, remember: RF ground is NOT necessarily DC ground. It CAN be, but often is not. That's where we must "fashion" a potential to ground that results in a 50 ohm path from the antenna to ground. I know, it doesn't make a whole lot of sense to those who may be new. This is where an antenna analyzer comes in VERY handy. Usually, at 27 MHZ, this is not necessary, the general rule being that the higher in frequency we go, the less we have to monkey with feedpoint impedance. This gets into the various values of resistance (feedpoint, radiation, and coil [if any] ) that are present in an antenna system. I don't have time to do that right now. After all, we have gotten off topic a bit and it won't help the fellow with his issue.

A coil that is IN the antenna itself is there for ONE purpose. It does NOT make your signal BIGGER or BETTER. It is there because the antenna is too LONG, & it has to be shorter due to physical limitations or height restrictions. In the case of lower HF frequencies (below 27 MHZ), some mobile antennas would have to be 60 feet long!!!!:unsure: We can't have that!

CWM
 
where is the ant. mounted? is this on a big truck or a pick up? if it's on a pickup the ant. needs to be mounted so that 2/3 of the ant. is above the cab. otherwise you can get a lot of reflection and i believe you will have high swr's.
 

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