signal impacts due to antenna locations and relation to other antennas

[DaveInDenver]

I wouldn't sweat it. It's always useful to keep a low and high gain antenna. Run the low profile, low gain, wider pattern antenna daily and if you're at the fringe of a path then you can always stick the high gain on to get the extra signal strength.

There's a bit more to selecting an antenna than just gain. How they will tune against your counterpoise (ground plane) might direct you towards one or another.

For example I'm currently running antennas at the base of my windshield on each side of the cowling on a little brackets. These provide very little plane under the whips and don't couple readily with the hood of my truck.

A 1/4 wavelength doesn't work well. It'll tune to a reasonably low SWR so the radio isn't going to be stressed but I know from experience and education it's not really tuning as a good antenna since it's probably mostly working against the coax shield. So I use 1/2 wavelength antennas that don't rely strongly on having a plane under them to work. I was kind of forced into the decision to use antennas I did because it's ultimately better to radiate all your RF energy non-ideally than to waste it heating your feed line.

One thing to note, it's always better to mount an antenna on the roof or middle of a truck lid no matter the type it is. Just that some don't rely on it to work so moving them off the ideal location results in less gain loss or tune-ability.

This graphic is for 800MHz so for a cell phone antenna but you get the idea. Height is key.



Your stacked array GMRS antenna on a fender might work just as well as a short 1/4 on the roof in most cases. You get whatever, say 6 dB of gain, which when you subtract roughly 3 dB for a less than ideal location means you still have gain over a no-gain reference. This is all part of the compromises you make.

What isn't indicated is the change in patterns of course. A 1/4 on the roof has practically no RF deafness in any direction or elevation. Depending on how you look at it is either always perfect or always terrible. A high gain on a corner has a distorted pattern that might leave large chucks of space with less than stellar performance.

In a generalization like this it would mean in the real world your truck turning 90 or 180 degrees right or left could vary the signal strength quite a lot when mounting using the non-ideal locations. In aspects where it does work it could work very well while in others it works very poorly.

 

[Chorky]

I wouldn't sweat it. It's always useful to keep a low and high gain antenna. Run the low profile, low gain, wider pattern antenna daily and if you're at the fringe of a path then you can always stick the high gain on to get the extra signal strength.

What isn't indicated is the change in patterns of course. A 1/4 on the roof has practically no RF deafness in any direction or elevation. Depending on how you look at it is either always perfect or always terrible. A high gain on a corner has a distorted pattern that might leave large chucks of space with less than stellar performance.

In a generalization like this it would mean in the real world your truck turning 90 or 180 degrees right or left could vary the signal strength quite a lot when mounting using the non-ideal locations. In aspects where it does work it could work very well while in others it works very poorly.

True, and since I have the double 5/8 and the 1/4 that came with the radio, mine as well use it... Now, based on the 'gain' graphic below, in your experience, is there even really a significant difference angle wise between a high/low gain antenna in terms of real world functionality? Only because sometimes theory and reality don't quite match up.

Its a great graphic. I look at it quite frequently actually. Here's a few more.
Standard gain differences - which are obvious. I don't know how accurate this is though.




Placement in relation to pattern




Lesser known stuff of angle - which probably only takes affect while driving/bouncing around




So, the conclusion I have come to, and please correct me if I'm wrong, but to keep things relatively simple without needing to do mass calculations for precision accuracy, so long as antennas of similar frequencies (so the GMRS, 2m/70cm, scanner) are sufficiently spaced apart due to potential for rx induced damage, antennas themselves are not a signal blocking/altering 'object' (like if you placed a 10' steel sheet in front of an antenna). And the CB and cell extender antennas are more or less 'invisible' to the other radios. So in other words, antenna(s) would not cause reduction in signal to/from the various radios, but antenna placement is the bigger issue/compromise. However, the proximity of GMRS and 2m/70cm antennas do need to be sufficiently spaced to prevent damage and signal distortion potential.

So since I have a full size truck, it seems to me I have almost sufficient 'roof' space to accommodate things? Possibly rearranging the original layout idea on post #1 to allow further spacing between the GMRS and a future 2m/70cm (which by the way is because the idea of APRS really sounds intriguing to me and something that would be fun to tinker with). So possibly the layout would be something like this: 1' from front of canopy is GMRS antenna (this would be due to the fact that the truck cab (even though there is about a 4" gap between cab and canopy) would in theory act as a ground plane for the GMRS). Something like 3' from front of canopy would be the cell extender. And something like 4' from rear of canopy would be the 2m/70cm antenna - which would give a significant distance between the GMRS and HAM. That leaves a new placement needed for the scanner, which could just go on the left front fender (opposite of the factory radio antenna) causing some blindness from the right rear, but depending on antenna height maybe not a whole lot. This would be to give the scanner sufficient distance from rx of GMRS or ham to prevent damage (I think), and all this would also mostly satisfy my OCD. Still undecided on the CB situation though - but to be honest, I'm still interested in the duals only because I already have both mounts, both antennas, and the right coax to run it (only need the spring) - but suppose I could mount just one of the two and deal with partial blindness on one side. Critiques on all of this thought though?

Now a follow-up question regarding antennas specifically regardless of band. If it was desired to have abilities to frequently lower antennas due to various height questionable situations. Would it be better/preferable/less of a compromise of signal quality/distance to A) use a spring (not sure if springs are even an option for anything other than a CB, I've never seen one). B) use a pivot mount, I have seen only a few online and usually only for lip mounts unless its built into the antenna. or C) just take the antenna off, which I really don't want to be doing on a routine basis. Option B would be preferable to me I think. Keeping in mind I really want to do a 'permanent' mount. So not a mag mount, and not a 'lip' mount. Just because I like clean installs, and not having wires hanging around. Also, a further question on signal. Lets assume the whip is on one side, mounted to the side of the canopy front (so basically right behind the rear window of the truck cab). IF the antenna was folded down (like you see old jeeps do at times), what does this do to signal? So if I was in an area with lots of low tree branches, and pulled and tied down the whip just to prevent all that clanging, but still wanted to talk to trail peeps. Seems like this could be quite an interesting signal pattern. Obviously strictly to one side of the truck and possibly shooting straight up into the air for the most part? Weird question I know...my curiosity gets the best of me.

Now, to up the complexity of discussion, lets use either the CB or GMRS just for example, since I dont yet have a ham. I'm sure there is a way to actually calculate estimated distances, or rather probably a computer program somewhere (i haven't found one yet) that can draw up a model, of the signal distance and quality of a given radio, with a given antenna, in a given location in relation to distance, height (angle), etc.? So that one could 'guestimate' how close to a friend or repeater they must be to chat. However, in the end there are so many environmental factors, it probably really doesn't matter other than satisfying my overly deep curios nature.

Apologies for the length and numerous more questions.

 

[prerunner1982]

Now a follow-up question regarding antennas specifically regardless of band. If it was desired to have abilities to frequently lower antennas due to various height questionable situations. Would it be better/preferable/less of a compromise of signal quality/distance to A) use a spring (not sure if springs are even an option for anything other than a CB, I've never seen one). B) use a pivot mount, I have seen only a few online and usually only for lip mounts unless its built into the antenna. or C) just take the antenna off, which I really don't want to be doing on a routine basis. Option B would be preferable to me I think. Keeping in mind I really want to do a 'permanent' mount. So not a mag mount, and not a 'lip' mount. Just because I like clean installs, and not having wires hanging around. Also, a further question on signal. Lets assume the whip is on one side, mounted to the side of the canopy front (so basically right behind the rear window of the truck cab). IF the antenna was folded down (like you see old jeeps do at times), what does this do to signal? So if I was in an area with lots of low tree branches, and pulled and tied down the whip just to prevent all that clanging, but still wanted to talk to trail peeps. Seems like this could be quite an interesting signal pattern. Obviously strictly to one side of the truck and possibly shooting straight up into the air for the most part? Weird question I know...my curiosity gets the best of me.

As far as lowering antennas I have long antennas mounted high and I ********** them on tree limbs daily and have for years with no problem, though I am sure eventually one will break. Now if it's a large branch almost dragging the roof of the truck, sure you would want to collapse them down. But for smaller branches even low hanging my antennas have enough give that I don't worry about it. Then again I also don't run expensive antennas.

There are some antennas with springs available for other antennas such as GMRS and Ham, no sure about cell booster.

Laying a UHF/VHF over and using it, it may be too close to the metal roof and too much signal will be reflected back right at the antenna.
If you were operating HF and folded a large antenna over the truck it would be NVIS (Near Vertical Incidence Skywave) which is used for short distance HF contacts.

 

[Pacific Northwest yetti]

My truck has a scanner, CB, Quadband and, booster.

CB antenna on bumper,

Quad band antenna drivers side a pillar

Booster antenna on headache rack- mag mount, short little things.

Scanner antenna- tucked in to the passenger side A pillar.

So far so good. I used to run dual CB's from the rear headache rack prior to the cabover camper and quad band.

 

[BigSwede]

Don't forget about trees. Not to be a killjoy over all this lovely tech talk, but if you ever bushwack down a half-overgrown logging trail those pretty roof antennas will be toast.

Or maybe you don't, in which case carry on.

 

[dstock]

I just ran into the GMRS/70cm interference issue having both antennas mounted to my front ARB bumper.

I will probably move the GMRS to the rear up on my AEV Tire carrier post since it's short and won't have a tree issue.

Ain't upgrades fun? There's always something to do!

 

[sonoronos]

Apart from moving around antennas, filtering and/or RF switches to provide isolation, there is also significant amounts of research in the topic of full-duplex wireless. Self Interference Cancellation and other techniques are not really practical today, but maybe one day something will be available to the general consumer market.