Mobile Ham antenna configs

[Red_Chili]

So I have a soft top.

And a sunroof on what little cab I have.

There is always the hood for a ground plane, but then the antennae (2M, 70CM) are in the field of vision.

Reading through the ARRL book, I see lots of interesting (and cheap, appeals to my inner cheapskate) antenna configurations,

Has anyone run screen as a ground plane in something like a soft top? Wire arrays? How about a loop antenna around the circumference? That would be too close to what little ground plane I have though wouldn't it?

Just kicking around ideas.

Learning about antenna design, it's amazing my CB works as well as it does. Hardly any ground plane to speak of, attached to the spare tire carrier.

Cool stuff. RF does really weird things.

 

[leiniesred]

MY CB antenna is mounted in the bed of my 4runner. It fits inside the hard and soft top. Optimal? no. It is corner mounted making it directional, plus 15 inches of it is reflected by the box, and the remaining foot gets reflected by the cab.

I need an antenna design that won't make my rig more than 1 inch taller than my roofline, otherwise, the truck won't fit in the garage.

How does my tiny XM antenna work? I like it.

Shoot, cops seem to get along with <1 foot antennas mounted on the trunk lid. I see like 3-5 antennae sticking up back there.

Back in the old jeep and bronco daze, I had the good ol' 20 foot whip. Works great as a CB antenna AND is good for knocking leaves, sticks, and dead raccoons into your hair. (chicks don't dig that.)

 

[Red_Chili]

It depends on the wavelength.

I definitely want to be set up for minimal SWR and a quality install for this rig though. Cheap and not subject to tree branches would be a bonus!

 

[DaveInDenver]

There seems to be a misunderstanding of what a ground plane is and why you care. You say you have the ARRL book, I assume it's the study manual for the test and not the ARRL Antenna Book.

If you guys want, I've posted bits and pieces about this stuff in other places and have been trying to compile that into a single file for reference (adding diagrams for clarity) that I was going to offer to Groucho for the class. So for the sake of conciseness here I will just say that there are physical reasons why a CB antenna is more tolerant of a bad local ground plane than your VHF and UHF radio will be. You can get a fair signal out with a CB with a poor vehicle ground plane where a 2m antenna needs more care (it's also easier to actually get a good 2m ground plane). You do have a ground always in every antenna that's not shorted, it's just how ideal the current path and incident wave are. In the case of a CB, the earth itself is your ground plane and that works fine, but the path back to the radio is not ideal necessarily.

The short answer is for you Bill is that your best option is right in the middle of the cab. Best option is to drill a hole, but I know you've said you don't want to do that. I have my ham antenna on my roof, slightly rear of center. I get good range out of it and even though your roof is a little smaller, it would be a very good ground plane option. Depending on who you reference the optimal ground plane size varies from about 1/4 to 1/2 wavelength, but in general you don't see any additional benefit beyond 1/2 wavelength. So in reality if you can get about 3 feet of sheet metal in all directions you will be about ideal for 2m. That is not really possible on most cars and so I tend to shoot for 1/4, so about 18" in any direction. That's pretty close on a soft top 4Runner.

I would suggest a magnetic mount. The quality of ham equipment is much higher than the CB stuff, so before you balk at that just realize that hams have the benefit of sharing a lot of gear with commercial gear, so mag mounts on balance tend to hold better and do less damage than the junk you get at Rat Shack.

It is possible to use the hood, but I find that distracting and not much more convenient. Other options will be roof gutters, door edges and on the bumper. I had good luck with my ham antenna on the bull bar (and still have the CB out there), although geeks like me can give you pages of reasons why that should be lousy. You do have a very directional pattern on the edges of the car, but it's workable.

 

[Red_Chili]

In the middle of my cab roof is... glass.
The biggest piece of existing sheetmetal is my hood. Fenders are 'glass'.
That's kinda why I was wondering about creating a ground plane in the soft top. I didn't know (until I read it in the ARRL test prep book) that this could be effectively done with screen or a wire array.
OR... coming up with an antenna that doesn't require a ground plane 'mirror'.
Whatcha think?

 

[Beater]

bill, I really think your taking that "idle hands" passage waaaay to far.

j

 

[DaveInDenver]

In the middle of my cab roof is... glass.
The biggest piece of existing sheetmetal is my hood. Fenders are 'glass'.
That's kinda why I was wondering about creating a ground plane in the soft top. I didn't know (until I read it in the ARRL test prep book) that this could be effectively done with screen or a wire array.
OR... coming up with an antenna that doesn't require a ground plane 'mirror'.
Whatcha think?

Duh, forgot about the sun roof. Does it happen to be that passivated glass? It might still work OK for a ground plane if it's conductive and got a good connection to the truck body.

To answer your question about the soft top. Yes, you can build a counterpoise into or on your soft top. This is actually a pretty common technique when dealing with fiberglass roofs and non-metallic bodied cars (Saturns, Vettes, etc.). You build a conductive plane under or over the composite using copper sheets, fine wire mesh, wire radials, etc. In the case of a 2m/70cm antenna that could certainly be a mesh ring under your antenna. I think the issue with a soft top is mostly getting a solid mechanical fit for the antenna (unless maybe you are thinking of a roll bar or something?).

You mention the so called no ground plane antennas. That's something of a misnomer, there is no such thing. But physically a 1/2 wavelength, half dipole, end fed antenna is a bit of an oddity in how it expects the return path to couple and so it appears to the radio that you don't need a ground plane. Some of this I'm sure Groucho will cover, but you don't get anything for nothing. Basically they do what they say, give you a low VSWR without a good groundplane, that is true. You give up some efficiency and change your directionality and pattern for it. It's certainly an option to keep in your back pocket.

 

[Red_Chili]

The soft top has a stainless frame that I could mount an antenna to - with a small sealed hole through the canvas. How large of a mesh ring would be needed to form an adequate plane? How do you figure the dimensions needed?

The downside would be interference with the rack, which I now don't use nearly as much (costs me 2mpg, believe it or not).
I could put it toward the cab and pick up somewhat of a ground plane from it. The top mount will be vulnerable to trees, and Miss Chili ain't the shortest Amazon in the herd.

I prolly oughta go by Radio Service Center and jaw.

 

[DaveInDenver]

The soft top has a stainless frame that I could mount an antenna to - with a small sealed hole through the canvas. How large of a mesh ring would be needed to form an adequate plane? How do you figure the dimensions needed?

There is some disagreement amongst references and engineers, but on VHF generally 1/2 wavelength is considered best and anything above 1/4 wave is perfectly sufficient. So since 2 meters is what you're shooting for and the other half of your dual band is shorter 70 cm, take 1/2 of 2m = 1m or about 3 feet. That is ideal. Since that is also essentially impossible, a realistic ground plane would be 1/4 of 2m = ~ 1.5 feet. So an 18" doughnut of mesh would give you a good ground plane on 2m and a great ground plane on 70 cm. If it had to be even 12" or 15", it would still be not bad on 2m and still pretty good on 70 cm. It's more important that you have some ground plane that's above maybe 12" or so, beyond that you are starting more into optimizing than just getting a good antenna.

The downside would be interference with the rack, which I now don't use nearly as much (costs me 2mpg, believe it or not).
I could put it toward the cab and pick up somewhat of a ground plane from it. The top mount will be vulnerable to trees, and Miss Chili ain't the shortest Amazon in the herd.

I prolly oughta go by Radio Service Center and jaw.

How about on the rack? A piece of thin sheet aluminum or copper mounted on the floor of the rack would be really good.

 

[Red_Chili]

The rack stays home most days. I haven't even fixed the towers since I ripped it off running Kane Creek with too much weight up top last year.

 

[Shark Bait]

Hey Dave,

While you're at it, what is the best mounting point for a 2m antenna in an FJ40? I was thinking of putting a tab on the roll cage somewhere. Maybe even a plate big enough for a mag mount. One for the CB on one side and one for the ham antenna on the other.

 

[Seldom Seen]

Don't get hung up on theory

You are looking for a problem that might not be there. Get a good NMO mag mount, slap it on the roof behind the sunroof and try it. If you plan on starting with 2m only, get a Larson NMO 150 and give it a go. I'm willing to bet a 6 pack it will work fine. If it doesn't or you want to optimize your investment and use 70cm, go with a Larson NMO 2/70.

The 2/70 is 1/2 wave end fed on 2m and in "theory" does not require a ground plane (manufactures other than Larson claim their 1/2 waves are ground independent) in practice, however, the ground independent thing only holds true for the exact freq the ant is tuned for. Move off that freq and ground current flows and the outer shield of the coax becomes part of the ant system (not good) But if you place the 1/2 wave over even a marginal ground plane, ground currents are not a problem because of capacitance coupling of the ant and the car body. The motorcycle guys love the 1/2 wave because of the lack of an adequate "ground plane" on their bikes.

In "theory" going from a 5/8 wave (like the NMO 150) to a 1/2 wave you give up some range for elevation, but in the real world you would be hard pressed to tell the difference.

When Groucho and I went up to Mt Bross, for the 14er event, we were using his home brew beam ant. At some point the coax connection came in contact with one of the driven elements of the beam. Had we been using something as cheaply made as a CB, the radio would have been toast but the "fold back" protection in the radio kicked in and cut back the output power (if we were able to measure it, I'm willing to bet it was only a few mw's) So in "theory" we were putting a couple of mw's into a dead short and yet we were able to transmit over 40 miles, go figure

Theory is great and interest in it will help you to advance in the hobby. But if you are using theory to find problems that ain't there, you'll never try or gain anything. Try it, if it don't work apply the theory in your trouble shooting thought process and then try something else.

It's a Hobby, have fun with it, don't get hung up on it.

 

[Red_Chili]

Who, me, hung up?
Just trying for doin' it right the first time.
So whatcha think about just plunking this NMO 150 thingie on my spare tire carrier then?

 

[Hulk]

It will be fine.

 

[Seldom Seen]

It will be fine.

yep

 

[Shark Bait]

So whatcha think about just plunking this NMO 150 thingie on my spare tire carrier then?

What?

 

[DaveInDenver]

What?

The NMO150 is a good 2m antenna. Whatcha confused about? It's that Bill's installing a dual band radio, probably. Yeah, I'd suggest putting a NMO2/70 on the tire carrier instead. A single band antenna will work, no problem, but you have to give up UHF transmitting.

Hey Dave,

While you're at it, what is the best mounting point for a 2m antenna in an FJ40? I was thinking of putting a tab on the roll cage somewhere. Maybe even a plate big enough for a mag mount. One for the CB on one side and one for the ham antenna on the other.

With VHF and UHF, there are two things you are thinking about, a decent ground plane and height. Of the two, height is probably the more important. A decent sized disc in the middle of the roll bar will be good. If you can swing it, 18" or bigger, but if it's a little bigger than a good mag mount it should work fine.

 

[Shark Bait]

Thanks Dave. I'm trying to figure out where to put antennas on my FJ40 that will work the best with the least amount of hassle. I'm thinking now for my Ham antenna, a mount on the top of the spare tire carrier is how I am going to go. Can I get away with a CB antenna next to it? I also have a nice Wilson mag mount CB antenna I could stick in the back of the tub on one of the wheel wells, or something. The trick, for me, is to find a place to mount stuff that will work with the soft top either on or off.

 

[Convert]

Thanks Dave. I'm trying to figure out where to put antennas on my FJ40 that will work the best with the least amount of hassle. I'm thinking now for my Ham antenna, a mount on the top of the spare tire carrier is how I am going to go. Can I get away with a CB antenna next to it? I also have a nice Wilson mag mount CB antenna I could stick in the back of the tub on one of the wheel wells, or something. The trick, for me, is to find a place to mount stuff that will work with the soft top either on or off.

Chris
I have my Ham and CB antenna's mounted on the spare tire carrier 6 to 8 inches apart and have seen no problems. I know a lot of people say not to mount Ham antenna's on the bumpers but I did not want to cut a hole in the roof. I have talked 30 to 40 miles on simplex rolling through canyons with the current setup.

 

[DaveInDenver]

With the interchangeable tops that does make things more complicated. You could do dual mounts, a tab on the roll bar and NMO in the hard top. Put a SO239 someplace and have pigtails on both mounts. So when you put the hard top on, you hook it up, soft top, vice versa. You could get real fancy and put in an antenna switch, even.

You can mount antennas for two different bands in close proximity, won't hurt anything RF-wise. As long as there's some spacing and both transmitters are not generating horrible out-of-band noise (keep 'em tuned!), the radios won't care that much. They might care if the whips were like an inch apart because no front end is perfect. That's not a realistic situation because...the whips must never touch.

During both receiving it might not hurt anything if they touched, I'd have to think about that. But if you happen to be transmitting when that happens you stand a good chance of killing the receiving one or maybe both radios. This would be particularly true if you were talking at 50W on the ham and the whips touched. You could say bye-bye to the CB receiver for certain.


Some informational geeky junk:

You can generalize your power and field strengths by using:
View attachment 7414
S = field power flux density (would be W/m^2 using unit below)
P = transmit power in watts
G = antenna gain
R = distance from center of antenna in m^2

Where gain G is found from your antenna gain in dBi:
View attachment 7415

To find the E (voltage) field strength:
View attachment 7416

At 1 foot, a 50W TX on 145MHz through a 1/4 wave (0dBd gain, i.e. 2.2dBi) antenna produces a power density of about 70.9 W/m^2, which would be an approximate field strength of 164 V/m.

Consider that modern (as in made since about 1940) radios can detect fields in the tenths of microvolts per meter (a FT-8800R can detect a signal as low as 0.2uV/m) and so you can see being a couple of feet away means the field will be many thousands or millions of times stronger than the weakest detectable signal. I'd think most any decent front end filter will be able to tolerate this sort out of band interference and still not too significantly affect the sensitivity of the radio for its in-band frequencies. However, if the whips touch then there's no free space and you get the full current (practically an infinitely strong field) going straight into the front end. Not a good situation pretty much no matter if it's 1W or 100W. No matter how good the rejection in on the radio, grossly overloading it will be bad. Also this is only true of wildly out of band signals. The front end of our mobiles is designed for wide band RX, which means they are looking for a signal from about 108MHz up to 1GHz. So doing this with a CB is OK since it's quite a bit below the range of the FT-8800R and so the designers (it's an assumption, but should be a safe assumption on our ham gear) will design the front end to just attenuate it and it never even makes it to the RF amp. If you put a pair of 2m radio antennas this close the field would be too strong for the RX and it might hurt it. You want to keep a lot more separation between same band TX and RX antennas.