Multiple antennas, mounting questions and minimum distance between?

[Martinjmpr]

Still trying to outfit my new-to-me 07 4runner.

Here's my situation: I have a HAM license and a 2m mobile (as well as a dual band HT.) But I really need CB as well since almost nobody that I travel with also has HAM.

On my 99 T4R I mounted my 2m to the transmission hump and my antenna was a Larsen glass-mount. My reasons for choosing a glass mount were simply that it would allow me to run the antenna without trying to figure out how to get through a body panel to the outside of the vehicle, and it was also "garage friendly." I never checked the SWR on the Larsen but I was able to hit repeaters pretty easily.

For my CB, which is a Midland 75-822 (which requires no "hard mounting") I was running a pretty cheap mag-mount on the center of the roof (and which I took off when not traveling.) TX and RX with this arrangement was never great and I don't like the idea of keeping the mag mount because it looks like crap and it runs the risk of scratching my paint.

If I understand, CB radios, because they are such a long wavelength, either require a long antenna (like the old 8' whips we used to see back in the 70's) or a good ground plane. Obviously the best ground plane is the center of the vehicle body, but this doesn't make for a garage-friendly setup (plus I am reluctant to drill a hole in the roof of my 4runner.)

So my next thought was a hood lip mount. I know a lot of folks use those, do they make for a decent ground plane? The advantage of the hood mount to me is that I could go with as long as a 3' antenna and still be able to fit in my garage.

Also thinking about running a hood mount on the other side of the hood for the 2m, so that's where my question comes in as to how far apart the two antennas need to be in order to keep one radio from interfering with the other (though I can't imagine any situation where I'd be using both at the same time, so maybe this is not a consideration?)

Does anybody else here run dual hood mounts, one for the 2m and one for the CB? Any problems or issues as a result of that?

Of course, I may just fall back to the glass mount, which I can put back on with a re-mounting kit from Larsen.

 

[1911]

If I understand, CB radios, because they are such a long wavelength, either require a long antenna (like the old 8' whips we used to see back in the 70's) or a good ground plane.

The band wavelength itself is not what determines the need for a ground plane, but rather what resonant multiple (or fraction thereof) of the wavelength is represented by the length of the antenna. For both 2m and 11m (CB) antennas, a 5/8-wave or 1/4-wave antenna will need a decent ground plane to radiate efficiently, but a 1/2-wave antenna will not.

So my next thought was a hood lip mount. I know a lot of folks use those, do they make for a decent ground plane?

Yes, the broad flat expanse of sheet metal (the hood and tops of the fenders) makes a pretty decent ground plane. Your radiation pattern will be elliptical with the antenna on one side or the other. Not a big deterrent in most situations.

Also thinking about running a hood mount on the other side of the hood for the 2m, so that's where my question comes in as to how far apart the two antennas need to be in order to keep one radio from interfering with the other (though I can't imagine any situation where I'd be using both at the same time, so maybe this is not a consideration?)

As long as they don't touch each other while they're both on and you're transmitting, you will be fine. I actually have both cb and 2m on at the same time fairly frequently in a trail group - cb as the lowest-common-denominator for the rest of the group and 2m to stay in touch with different trail groups and/or base camp, and I also beacon APRS all the time.

Does anybody else here run dual hood mounts, one for the 2m and one for the CB? Any problems or issues as a result of that?

Neither of mine are on the hood, sorry.

Good luck with your installation.

 

[4x4junkie]

The length of each antenna is going to have some bearing on how far apart they can be.
If your CB antenna towers over the VHF (2M) antenna, it's not likely to be adversely affected by the VHF antenna's presence. At the same time the VHF antenna won't be bothered by the CB antenna if it's at least it's own length away from the CB antenna. However if both antennas are roughly the same length as each other (as would be with a coil-loaded CB antenna), the one operating on the longer wavelength (CB) is much more likely to be affected by the other antenna if that antenna is less than ¼ wavelength from it.

What you might want to get for your CB ant is a simple quick-disconnect mount, you just push down & give it a quarter turn and it comes right off. This way you can use an antenna that has a decent enough length that it'll work well (and not be bothered by your VHF ant), yet allowing you to remove it for your garage.

 

[Shawn686]

I am a comms noob.

I just got a CB and was going to do a roof mount nmo but that would mean I cant run a RTT up there. How long is a 1/2 wave length antenna? If I am reading the above correctly 1/2 wave means I can mount on my hood lip without much effect on the radiation pattern and not have to worry so much about ground plane.

Shawn

 

[1911]

I am a comms noob.

I just got a CB and was going to do a roof mount nmo but that would mean I cant run a RTT up there. How long is a 1/2 wave length antenna?

CB is +/- 27 MHz, and the wavelength is 11.1 meters or 36.4 feet, so 1/4-wavelength is 9.1 feet. That is why some folks run a simple whip 9 feet long. But if you don't want to do that, then Firestik and Wilson, etc. make antennas with either a base coil or a fiberglass rod with wire wrapped around it a bunch of times, so that you can get 9 feet of wire in a much shorter package. You can get them in 2', 3' or 4' lengths. The different lengths are electrically identical, but the taller the antenna is (the higher up in the air it is) the better the range.

If I am reading the above correctly 1/2 wave means I can mount on my hood lip without much effect on the radiation pattern and not have to worry so much about ground plane.

You are talking about two completely different things. 1/2-wave means that you don't have to worry about ground plane (though your hood + fenders make a pretty good ground plane, you could run a 5/8-wave there if you want).

In theory the radiation pattern would be circular (equal in all directions), if the antenna were located in the center of a perfect ground plane, but trucks and cars make very imperfect ground planes, even when there is a horizontal expanse of sheet metal (such as a roof or a hood). When you mount the antenna on one side or another, the radiation pattern is pulled into an ellipse with the longer lobe of the ellipse pointing over where the most ground plane (sheet metal on a car) is. That's why the best place (for performance anyway) for any mobile antenna is in the very middle of the roof. In real life, with a 4-watt cb, the radiation pattern makes no practical difference. On the roof is still better though because the antenna will be higher and range will be better.

If all you need is to talk to your trail group a few trucks up and a few trucks back, then a well-tuned antenna almost anywhere on the outside of the vehicle will do. But then we get back to ground plane, because many (most?) cb antennas sold are 5/8-wave and may not tune up well on a bumper or a spare tire mount.

 

[Shawn686]

OP sorry to high jack.

I currently am just running a wilson little wil mag mount and am getting ok range. I just got that as the place that i got my radio had them in stock and it was a good temp solution until I figured out where and what to permanent mount.

I am thinking of when I make the permanent mount that I want to run wilson 1000 or 5000 nmo so its easy to remove for car wash etc. But don't know which would give the best results and what length of whip to use. I have no issue running a long whip so length is not an issue. My radio also receives the weather channels if that makes a difference.

http://www.wilsonantenna.com/base_load.php

So since the CB is only 4 watts I don't need to worry about ground plane as its not powerful enough for it to make much of a difference. And the higher/longer the antenna is the better the range will be.

A 1/4 wave antenna is 9.1 feet or 109.2 inches, if I go 1/4 wave how do I tell how long to make the whip if I use a wilson 1000 or 5000? It doesn't list the length of the base load. If I get what you said about base loads being just wire wrapped up to save length. So to get the whip length you would subtract the based load from the total of the wave you are wanting to use, correct?

Shawn

 

[1911]

I am thinking of when I make the permanent mount that I want to run wilson 1000 or 5000 nmo so its easy to remove for car wash etc. But don't know which would give the best results and what length of whip to use. I have no issue running a long whip so length is not an issue.

The Wilson 1000 and 5000 both have base-loaded coils as I mentioned above, so you need to use whatever whip comes with them since the proper length for the resonant multiple (probably 5/8-wave I bet) is built into the design and includes many windings in the coil.

If you want to run a plain 1/4-wave whip, it has no coils on the bottom or the top, it is just a piece of steel rod 9.1 feet or 109 inches long. Mount it on a big spring and bolt the spring on your truck body somewhere. All the sheet metal on the truck will constitute your ground plane. Tune it with an SWR meter by cutting it to length.

My radio also receives the weather channels if that makes a difference.

No, it doesn't make a difference since you are only receiving the weather bands (which have a wavelength of 1.85 meters BTW) and not transmitting. You could hear them with a coat hanger for an antenna.

So since the CB is only 4 watts I don't need to worry about ground plane as its not powerful enough for it to make much of a difference.

No, it's the radiation pattern of the antenna that you don't need to worry about. Whatever antenna you use still needs to be tuned, to radiate efficiently and not damage your radio, and depending on your choice of antenna it may need a ground plane to tune-up. If your antenna isn't tuned you will not get anywhere near the range it is capable of, and if it is badly out of tune it will reflect the transmitted power from the radio back as heat and shorten the life of the radio.

And the higher/longer the antenna is the better the range will be.

Correct; all else being equal.

A 1/4 wave antenna is 9.1 feet or 109.2 inches, if I go 1/4 wave how do I tell how long to make the whip if I use a wilson 1000 or 5000? It doesn't list the length of the base load. If I get what you said about base loads being just wire wrapped up to save length. So to get the whip length you would subtract the based load from the total of the wave you are wanting to use, correct?

Yes, that is why you need to use whatever (length of) whip is supplied by Wilson for those antennas as I explained above.

 

[4x4junkie]

The Wilson 1000 and 5000 both have base-loaded coils as I mentioned above, so you need to use whatever whip comes with them since the proper length for the resonant multiple (probably 5/8-wave I bet) is built into the design and includes many windings in the coil.

They effectively operate as a ¼ wave.

Also, most full-length whips are 102".

 

[1911]

Also, most full-length whips are 102".

Because they're almost always mounted on a spring, and the whip needs to be cut to account for the length of the spring since it is conductive and becomes part of the antenna.

 

[Shawn686]

Cool, Thanks

I get it now, my radio also has a built in SWR meter so I am good to go on checking that.

Shawn