High Resistance on CB and 2M Mobile Antennas

[AndrewGS]

I've got a problem that I can't figure out. I have high resistance, by about 20 ohms, on both my CB and 2M mobile antennas. Both are mounted on 3/8x24 stake pocket mounts on my pickup. There are no shorts in the feedline, no shorts on the mounts and they are grounded to the frame with grounding strap. I've also grounded the bed to the frame with grounding strap.

Any recommendations?

 

[Robb]

OK
So - your seeing a 70 ohm load - rather than 50 ohms?
Hmmm...That works out to a SWR of 1.4 - if I have done my math correctly.
Is that your reading - 1.4?

Its not that bad; have you changed anything in your system?
Check for rust in any joints?

 

[Beetle]

What are you using to measure this resistance, and from which exact point to which exact point? How long is the feedline, and what kind is it?

 

[W5LZ]

Not all that unusual really. Fairly 'standard' for 1/4 wave antennas (all of them really). If you really want to get them closer to 'perfect', I'm sure it can be done. Expect the usual 'problems', the feed line isn't the answer, and adjusting impedances with a typical commercial antenna isn't exactly the easiest thing to do sometimes (especially with only an SWR meter).
- 'Doc

 

[AndrewGS]

OK
So - your seeing a 70 ohm load - rather than 50 ohms?
Hmmm...That works out to a SWR of 1.4 - if I have done my math correctly.
Is that your reading - 1.4?

I haven't really tried to adjust the SWR on the 2M antennas yet, but I have on the CB antenna using the radio's built-in meter and it gets down to an acceptable level, but I have a weird issue where the noise level fades in and out over time, as if it were an internal radio problem. Receiving is no problem on the 2M, however.

Its not that bad; have you changed anything in your system?
Check for rust in any joints?

It's all new. I redid everything because I thought it was the feedline and lack of proper grounding.

What are you using to measure this resistance, and from which exact point to which exact point?

MFJ Analyzer at the radio side.

How long is the feedline, and what kind is it?

CB is about 18ft. and 2M is about 24ft. of RG-8.

 

[Beetle]

MFJ Analyzer at the radio side.



CB is about 18ft. and 2M is about 24ft. of RG-8.

OK - "typical" R value for a mobile antenna in the 1/4 wavelength area is anywhere from about 25 ohms up to maybe 35 ohms. If your ground plane is adequate for the frequency in use, there shouldn't be a problem. You'll have to adjust the antenna itself for best SWR.

I think the problem might be in the way you're interpreting the readings on the analyzer. That "20 ohms" is NOT a DC resistance, like you'd get with a multimeter (and the way your question was posted, I thought that was what you meant). All antennas have a resistive value and a reactive value. The idea is to reduce the net reactance to zero, at which point the antenna is "resonant": inductive reactance is equal in magnitude and opposite in sign to capacitive reactance, therefore net reactance = zero.

The resistive component will be what it will be. Getting R close to 50 ohms while keeping X close to zero...that's the goal.

 

[Captain Kilowatt]

Which stake pockets are you using? If you are using the ones directly behind the cab then the antennas may be too close to the cab to tune right. then again some guys have no problem with using them. Such is the joy of mobile installations. What works for one may not work at all for another. Lots of variables with mobile installs.

 

[Moleculo]

Which stake pockets are you using? If you are using the ones directly behind the cab then the antennas may be too close to the cab to tune right. then again some guys have no problem with using them. Such is the joy of mobile installations. What works for one may not work at all for another. Lots of variables with mobile installs.

My thoughts, too.

You said you were using an analyzer, but you didn't tell us what the readings are?

 

[AndrewGS]

The analyzer read about 70ohms at about a 2:1 SWR.

Here are a few photos of my install.

As you can see, they are mounted on the rearward pockets. Also notice that the mounts are aluminum. Could that be a problem?

 

[W5LZ]

I honestly don't think that aluminum is going to make any difference at all. I did notice that there appears to be a plastic(?) insulator under that stake pocket mount. And assuming those 'bolts' on top is what's holding the whole mess in the pocket, I'm also assuming that the connections from feed line to mount are done correctly and the braid really is going to ground at the mount. I don't have any reason to think it isn't, but having never seen that mount before, what do I know about it? Just a thought.
And like someone said, making a compromise between resonance and a little bit of reactance (that 'X' number) to get the 'R' value closer to 50 ohms is pretty common. Try to keep that 'X' number low, and just don't worry about the 'R' value that much. I figure it'll do just fine the way it is. Wanna fiddle with it a bit? Have at it! The ideal is as small an 'X' number as possible, and something at least close to 50 for the 'R' number. It'll never be perfect... for long.
- 'Doc

 

[Captain Kilowatt]

What kind of antennas are they? Make and model?
The CB antenna isn't one meant to be run in pairs with a phasing harness is it? Running just a single one alone will cause the impedance to be around 70-100 ohms. Also what is the little black pigtail wire on the bottom of the CB antenna for? It looks like it connects the hot side of the antenna to something. The hot side of the antenna should be connected to the coax via the 3/8-24 stud. What brand'model are the stake mounts as well?

 

[Moleculo]

Also what is the little black pigtail wire on the bottom of the CB antenna for? It looks like it connects the hot side of the antenna to something.

That looks like a Wilson Silverload to me. They have those pigtails. I've never been able to get the SWR very low on one with that connected. I actually have one of these on my RV. That looks like the 3' length model, right? I couldn't get the SWR lower than about 2:1 on mine, either. Using my analyzer, I found the antenna was just too short. I used a piece of coat hanger and replaced the little adjustable stinger, making it longer. I had to poke through the little rubber cap. I also had to disconnect that pigtail. Doing these two things got the SWR down to where it was acceptable to me.

 

[AndrewGS]

I honestly don't think that aluminum is going to make any difference at all. I did notice that there appears to be a plastic(?) insulator under that stake pocket mount. And assuming those 'bolts' on top is what's holding the whole mess in the pocket, I'm also assuming that the connections from feed line to mount are done correctly and the braid really is going to ground at the mount. I don't have any reason to think it isn't, but having never seen that mount before, what do I know about it? Just a thought.

Yes, the two bolts hold a retainer that keeps the mount on the truck. The ground is screwed to the retainer. The mounts are custom made.

And like someone said, making a compromise between resonance and a little bit of reactance (that 'X' number) to get the 'R' value closer to 50 ohms is pretty common. Try to keep that 'X' number low, and just don't worry about the 'R' value that much. I figure it'll do just fine the way it is. Wanna fiddle with it a bit? Have at it! The ideal is as small an 'X' number as possible, and something at least close to 50 for the 'R' number. It'll never be perfect... for long.

I might try moving the ground from the mount retainer to the outside of the S0239 on the stud mount. If that doesn't change anything then I'll just set the SWR the best I can and forget it.

What kind of antennas are they? Make and model?

The CB antenna is a Wilson FGT-3 Silver Load 3ft. fiberglass loaded whip and the 2M antenna is an MFJ 1414 dual-band.

That looks like a Wilson Silverload to me. They have those pigtails. I've never been able to get the SWR very low on one with that connected. I actually have one of these on my RV. That looks like the 3' length model, right? I couldn't get the SWR lower than about 2:1 on mine, either. Using my analyzer, I found the antenna was just too short. I used a piece of coat hanger and replaced the little adjustable stinger, making it longer. I had to poke through the little rubber cap. I also had to disconnect that pigtail. Doing these two things got the SWR down to where it was acceptable to me.

Correct. It's supposedly a matching transformer to correct impedance, which is my problem. On the SWR meter it actually shows that it's too long, but I'm almost out of room on the stinger.

 

[Moleculo]

On the SWR meter it actually shows that it's too long, but I'm almost out of room on the stinger.

So you've pushed it all the way in and it still shows too long? FWIW, the 4 foot fiberglass Wilson & Firestik antennas are a lot easier to tune than the 3' ones.

One question, I'm assuming that when you grounded the mounts, you scraped away some of the powder coat on the frame where you attached the strap?

 

[Robb]

Is it also possible that the 2m antenna is too close and detuning each other?
They are almost the same height and not quite a 1/4 wave apart for 11m...
Just a thought...