Lots of antenna tuning questions - mag mount especially...

[Martinjmpr]

More questions from a relative nOOb to the Ham world:

Is it possible to tune a magnetic-mount antenna? I heard from someone that you can "tune" these antennas to a better SWR by adjusting the length of the feedline - does that make sense? How does adjusting the length of the feedline affect the SWR?

My question applies equally to CB and HAM (2m/440 dual band) antennas. I have a mag mount for my CB and may get one for my HT if and when I get it. Even though I have my Ham license I like the idea of having a CB when off-road just because so many other people use it.

But I have reason to believe my signal strength is not that great and for that I blame my cheap mag-mount antenna (last run I was on I sometimes could not talk to the vehicle in front of me.)

Oh, and before someone says it, yes, I understand that a tuned, fixed mount is better than a mag mount but I may be taking a short notice trip soon and "good enough" right now is better than "perfect" at some future date. Also I park my 4runner in a garage so a fixed mount would have to go on a bumper or fender - no roof mounting as it would not clear my garage door, even with a fold-down.

Are mobile antennas typically tuned by adjusting the length of the antenna? Reason I ask is because on a lot of mobile antennas it doesn't seem like it would be that easy to do. I do remember having an old dual-band antenna that came in two pieces, and you could slip the upper piece up or down an inch or so, then tighten it with an allen wrench. But it seemed like a pretty small area of adjustment - would an inch be enough adjustment for a dual band like that?

Are there SWR meters that will work on multiple freqs? Or do I have to get separate meters for CB and 2m/440? And how difficult is it to tune a multi-band antenna like a 2m/440 dual band?

BTW, I realize I could ask these questions on a HAM forum, but I guess I feel more "comfortable" here, since I'm talking with people who understand what I will be using the radio for, i.e. inter-vehicle communications while on off-road trips.

Thanks in advance for any help! :ylsmoke:

 

[SP Jon M3]

From what I've seen, most/all of the commercial brand 2m/70cm antennas are tuned from the factory to have a low SWR. So tuning is not required.

Extending/Shortening the coax will not (preceptivly) change your SWR.

The proper way to tune an antenna is by changing the length of the antenna wire to match the wavelength of the frequency you're operating on.

Yes, MFJ 259b will work on 2m and HF (this includes 11m which is the wavelength CB is on), the 269 will work on 2m/70cm and HF.

You should look at a Diamond K400 mount along with a NR770HB antenna (fold over). I have the same issues with having a low garage, my antenna folds down below the height of the roof rack.

You can see the dual band antenna on the back of my truck, the fold-over point it right at the top of the bulkiest part of the antenna.

 

[Tennmogger]

More questions from a relative nOOb to the Ham world:

"Is it possible to tune a magnetic-mount antenna?

>> Yes, you can tune a mag mount antenna. Most new antennas cover a range of frequencies and REQUIRE you to tune it to your specific frequency. There's usually a length chart included with new antennas. Normal procedure is to cut a little long first, measure SWR, then cut more if required (always cut off the big end, grinding is better. Notch it and break it). You can get an idea where the antenna is tuned by checking SWR at the top and at the bottom of the band. If the SWR is better at the low end, then the antenna is still too long.


"I heard from someone that you can "tune" these antennas to a better SWR by adjusting the length of the feedline - does that make sense? How does adjusting the length of the feedline affect the SWR?

>>>Cutting the coax doesn't tune the antenna but does change what the radio sees. That might possibly help only because the radio may be happier and put out more power, but the antenna won't be improved. Ideally the coax should be an odd multiple of a quarter wavelength at the frequency of operation, compensated for the velocity factor of the cable. That way the SWR at the radio will be the same as the SWR at the feedpoint of the antenna.

>>>Forget all that and just tune the antenna for best SWR at the radio, using an SWR meter (or forward/reverse power meter). If you take care to tune the antenna, the cable will have less effect.

"My question applies equally to CB and HAM (2m/440 dual band) antennas. I have a mag mount for my CB and may get one for my HT if and when I get it. Even though I have my Ham license I like the idea of having a CB when off-road just because so many other people use it.

"But I have reason to believe my signal strength is not that great and for that I blame my cheap mag-mount antenna (last run I was on I sometimes could not talk to the vehicle in front of me.)

>>You should be able to talk to the vehicle in front of you even with a dummy load and no antenna. Something is bad wrong. Maybe a shorted coaxial connector, wrongly mounted antenna??

"Oh, and before someone says it, yes, I understand that a tuned, fixed mount is better than a mag mount but I may be taking a short notice trip soon and "good enough" right now is better than "perfect" at some future date. Also I park my 4runner in a garage so a fixed mount would have to go on a bumper or fender - no roof mounting as it would not clear my garage door, even with a fold-down.

>>>There's nothing wrong with a mag mount. There's always a little trade-off to a fixed mount but a mag mount can serve you well. Just tune it in the position it will be used. For garage entry, consider a 1/4 wave whippy whip antenna for 2 meters, top dead center on the vehicle. It'll flex for most obstacles. For the CB, mount a short fiber glass antenna down low to clear the garage. Both those will work for trail obstructions, too. BTW, a dual band whip with the coil in it will be a limb trap.

"Are mobile antennas typically tuned by adjusting the length of the antenna? Reason I ask is because on a lot of mobile antennas it doesn't seem like it would be that easy to do. I do remember having an old dual-band antenna that came in two pieces, and you could slip the upper piece up or down an inch or so, then tighten it with an allen wrench. But it seemed like a pretty small area of adjustment - would an inch be enough adjustment for a dual band like that?

>>> for 2 meters an inch is a lot. Follow the antenna mfg instructions to tune a dual band antenna.

Bob

 

[1911]

More questions from a relative nOOb to the Ham world:

Is it possible to tune a magnetic-mount antenna?

It is if the metal whip is held in place by a set screw (like my Hustler mag mount); then you can raise or lower (or cut) the whip to move the center point of perfect resonance and the range of acceptable SWR up or down in the frequency spectrum.

I heard from someone that you can "tune" these antennas to a better SWR by adjusting the length of the feedline - does that make sense?

No. Use the shortest feedline of the correct impedance that will fit your installation.

How does adjusting the length of the feedline affect the SWR?

It really doesn't, though in some circumstances you can use the feedline like a counterpoise, if there is no ground plane.

My question applies equally to CB and HAM (2m/440 dual band) antennas. I have a mag mount for my CB and may get one for my HT if and when I get it. Even though I have my Ham license I like the idea of having a CB when off-road just because so many other people use it.

But I have reason to believe my signal strength is not that great and for that I blame my cheap mag-mount antenna (last run I was on I sometimes could not talk to the vehicle in front of me.)

Not sure I understand what your question is here.

Oh, and before someone says it, yes, I understand that a tuned, fixed mount is better than a mag mount but I may be taking a short notice trip soon and "good enough" right now is better than "perfect" at some future date. Also I park my 4runner in a garage so a fixed mount would have to go on a bumper or fender - no roof mounting as it would not clear my garage door, even with a fold-down.

I'm still running a mag mount for 2M/70cm on one of my 4WD trucks (an FJ Cruiser) and it works great; better range by far than the other truck with a permanent NMO mount, but that has more to do with antenna location and ground plane than whether they are mag mount or not. Get a mag-mount with an NMO on the base and then it take only a few seconds to unscrew the antenna when you enter or exit a garage.

Are mobile antennas typically tuned by adjusting the length of the antenna?

Yes, that is the only way to tune any mobile antenna.

Reason I ask is because on a lot of mobile antennas it doesn't seem like it would be that easy to do. I do remember having an old dual-band antenna that came in two pieces, and you could slip the upper piece up or down an inch or so, then tighten it with an allen wrench. But it seemed like a pretty small area of adjustment - would an inch be enough adjustment for a dual band like that?

If your ground plane is adequate for that antenna, then yes.

Are there SWR meters that will work on multiple freqs?

Yes.

And how difficult is it to tune a multi-band antenna like a 2m/440 dual band?

All multi-band antennas are a compromise of some sort. With some cleverness and a coil, you can make it 1/4-wave on 2m and 1/2-wave on 70cm, or 1/2-wave on 2m and two co-linear 5/8-wave on 70cm. But you're not going to get the same performance that you could out of a dedicated single-band antenna on either. The compromises work just fine for most people though. To answer your question, a lot of dual-band antennas are not adjustable for length and hence can't be tuned, but if they have a whip held by a set screw then you have some adjustment. They are more difficult to tune because altering the length will affect both bands.

 

[AA1PR]

also do not exceed the power rating of the antenna when say using a 2m 5/8 wave mag mount on 6m

experiement & have fun

 

[Scoutn79]

I didn't notice what length antenna you are running and where it is mounted.
If you have a steel roof and a 5/8 wave mounted in the middle of it you should be able to reach out 60 miles or so with a 50-60W radio. At least on flat ground I was able to and that was the longest test I have tried. If however you have a 1/4 wave mounted on a mirror or hood mount you are going to be limited as the vehicle will block the signal from going through it to some degree.
For reference I have a 60W mobile with a mag mount 5/8 wave antenna mounted on the mirror with a steel top on the Scout so I still have a good portion of the antenna above the roof.
Darrell

 

[DaveInDenver]

No. Use the shortest feedline of the correct impedance that will fit your installation.

It really doesn't, though in some circumstances you can use the feedline like a counterpoise, if there is no ground plane.

Sometimes the shield of the coax will be the current return path and that's even sometimes fine. Presents a number of problems when it does, though. In general you want to choke these currents at the antenna feedpoint, but doing so with a poorly designed antenna can impact how much signal you radiate.

All multi-band antennas are a compromise of some sort. With some cleverness and a coil, you can make it 1/4-wave on 2m and 1/2-wave on 70cm, or 1/2-wave on 2m and two co-linear 5/8-wave on 70cm. But you're not going to get the same performance that you could out of a dedicated single-band antenna on either. The compromises work just fine for most people though. To answer your question, a lot of dual-band antennas are not adjustable for length and hence can't be tuned, but if they have a whip held by a set screw then you have some adjustment. They are more difficult to tune because altering the length will affect both bands.

It's not always necessary to tune each band independently for simple dual band antennas. Larsen's standard 2/70 has enough bandwidth that when you tune for 2m the 70cm relationship will be close to resonance as well. The 2m antenna is a 1/2w.l. electrically and is physically just a little short (~33") using a small amount of inductance in the middle. At 70cm the inductor works like a 1/4w.l. electrical stub and so the whip looks like co-phased, collinear 1/2w.l. elements (each ~14") stacked and should show about 2dB of gain over a single element.

It's not pushing the envelope or trying to do anything fancy, it's fundamentally sound and works fine. Depending on the installation, the 70cm side might resonant a little below the middle of the ham band, but mine tuned for ~146MHz hits sits at (IIRC) 1.4:1 at 445MHz right in the middle of my cab roof. When you try and get more fancy, like using collinear arrays on both 2m and 70cm to increase gain, then you will have to tune each separately most likely. IMVHO it's not worth the hassle for a couple of dB more gain. Those antennas often use tiny coils (very lossy), have touchy tuning, narrow bandwidth.