signal impacts due to antenna locations and relation to other antennas

[Chorky]

Questions regarding antenna locations and impacts to overall signal quality/shape, and impacts to other antenna/signals. I have done some reading, researching, and questioning, but have found some conflicting info. So hoping I can get some clarification here before drilling holes...since sometimes you can't really go back after that.


My application/plan:

• Full size crew cab truck with alum/steel contractors style canopy.
• CB w/2 whip's - I tow
• cell extender
• GMRS
• Future 2m/70cm dual band
• Scanner
• Considering multiple radio's and compromises, the plan would currently be (all on the canopy, to prevent an 'oops' on the truck body as a canopy is replaceable): cell extender center and 1' from front of canopy, scanner center and 3' from front of canopy, GMRS center and 5' from front of canopy, 2m/70cm center and 7' from front of canopy - so all antennas are center canopy, evenly spaced apart. CB would be dual whip's, ball mounted to either side of the canopy, either at 1' from the front, or 1' from the rear.

My gathered info thus far, and questions:
To my understanding, and quite obvious, is signal quality depending on antenna location, mount type, material used, length of wave, etc. However, what I am most interested in is specifically antenna location and its effect on other signals and its own signal. Of course center roof top is optimum for a given single antenna - I have multiple.

• Considering say the GMRS and 2m antenna being only 2' apart (and just for simplicity assume they both are the same length/height, even though that is not accurate, humor me), will for example the 2m antenna being 2' behind the GMRS prevent the GMRS from transmitting well behind the truck? Information I have collected says no, but a friend who used to do professional mounting for government vehicles using multiple antennas/frequencies says the antenna (regardless of frequency) will interfere and thus cause reduced range rearward - for the GMRS in this example..., or the cell extender being up front would prevent the GMRS from a good signal going forward, for another example...
• To complicate further, if this be the case, if the CB's were upright and on the front of the canopy, would that mean they would essentially prevent all transmit signal forward of the truck from any antenna behind them?
• If this is in fact the case, how does one mitigate these effects working with so many radio's?
• if this is in fact not the case, is the current plan/layout outlined above the optimum compromise, or is there another that may be better to give optimum even signal on all sides (considering specific direction of other parties in relation to the truck may not be known right away).
• Considering the 2 CB whip's, would I be correct in that mounting the whips on either side of the front 1/4 of the canopy would have a more even 'oval' of a transmit, whereas if they were mounted on the sides of the rear 1/4 of the canopy then transmitting rearward of the truck would be limited due to little ground plane (canopy) being back there, yet transmit distance forward would be extended?

Looking forward to some input since information I have found seems conflicting at times. Thanks in advance!

 

[sonoronos]

inactive (non-radiating) elements (aka antennas) have, for all practical purposes, very little effect on a radiating element. One exception would be if the other elements were part of a single antenna system, but that doesn't seem to be the case here.

As frequency increases, then the effects can be greater (relatively) in the near field. But for far field, things in the sub-gigahertz range...best not to worry...

If you were designing a radar or imaging through phase-locked receivers, the discussion might be different...

 

[prerunner1982]

Your mounting sounds aesthetically pleasing.
The multiple antennas shouldn't cause any issues in relation to the others, performance wise. The only issue may be GMRS and the Dual band antenna being next to each other.
GMRS and 70cm are close in frequency and not only may it distort the radiation pattern it will likely overload the front end of the other radio when transmitting. At least 1/4 wave length apart is advisable though my APRS antenna was all the way across (diagonally) the vehicle from my AM/FM stereo and it would still overload it when beaconing.
If all your radios are mounted near each other you could mount the antennas how you want and if you have an issue you could swap antennas and connect the coax to a different radio.
You are correct on the CB whips, more center of the vehicle would be a more even oval, rear of the vehicle would be skewed towards the front.

By Canopy I assume you are referencing to a shell over the bed? Metal or fiberglass?
Fiberglass will require all sorts of bonding for adequate grounding.

 

[DaveInDenver]

The antennas whips themselves aren't really a concern but you do need to be aware of the field strength impact on radios. You can cause anywhere from no effect all the way to damaging front ends depending on configuration of bands and antennas.

You need same band radios connected to antennas that are as far apart from each other as possible, for example MURS and 2m ham or GMRS and 70cm ham. Various radios have different susceptibilities.

A CB or 160-to-10m ham station antenna can probably be safely near a VHF or UHF ham station antenna. That's not always the case. Some CB radios have weather band receive capability and being very near a VHF station might be damaging, for example.

Regardless, putting same band stations near each other is something of compromise for mobile. Say perhaps you have a VHF voice radio and an APRS radio you will never get enough separation on a vehicle that one will not affect the other in performance. When a 50 watt radio is transmitting you will overload an APRS radio and make it momentarily unable to hear anything else.

Even if you terminate a 50 watt (or even 5 watt) radio into a dummy load that has no radiating ability a radio across the room will hear it. It's possible to use 5 watts to talk hundreds of miles on HF with highly efficient antennas and good conditions (called low power or QRP operating). So ultimately you have to do your best and live with any desensitizing and just try to avoid damaging anything.

My old Uniden CB radio would blank whenever I transmitted on 2m ham at full power with the antennas both on the bull bar (maybe 2 feet of separation). The radio lasted darn near 20 years in my use and did finally give up, although I have no reason to suspect ham radios being within the near field had anything to do with it. Although I didn't do an extensive troubleshooting my guess is 20 years of vibration, dust, temperatures has more to do with it than anything.

I never actually checked it's receive sensitivity to see if long term it was being damaged but I couldn't say I ever became noticeably more deaf either. My new CB radio seems more immune to out of band noise, so there's at least some improvement with 30 years of development.

Something to think about is it's possible to employ horizontal or vertical spacing. If you mount one antenna on a roof and another on a fender lip you might be able to reduce the near field strength fairly significantly.

Personally I would be cognizant of how you locate higher power radios' antennas, e.g. 50 W ham or GMRS radios, particularly relative to other ham, GMRS and scanner antennas, and generally to all radios. Beyond that I would mostly concern myself with making sure they can't physically touch.

BTW, trying to phase two mobile CB antennas is at best going to be frustrating (if you actually achieve it) and in reality moving going down the road is basically impossible without active phasing units. I'd just run one well installed and tuned CB whip, that will be more efficient and work much better in the long run. Commercial broadcasters sometimes employ phased antennas but they have the benefit of a fixed transmit frequency into radiators that can be located precisely and not move and flop around. Hams do tinker with this but most of the time it's to optimize stationary (at home) receive performance and I'm not aware of many that attempt it for transmitting, certainly not with any significant power. Most of the time shaping signals is limited to Yagis with only a single active element.

 

[Airmapper]

Watching this discussion with interest as my truck is becoming a porcupine of antennas as well.

I see a problem forming on my rig as I typically run APRS on my dual band D710, in remote areas definitely pumping out at 50w, and am installing a 15w GMRS rig.....

I will add to this discussion that I had tuning issues with my CB whip because of my broadcast FM receiving antenna (vehicle OEM antenna) was too close. I still don't have an ideal SWR but good enough for trail comms, but with the vehicle antenna in place it woulden't tune. I currently remove the vehicle antenna when using my CB, fortunately for me that is not frequent and usually only on the trail with friends, rest of the time the CB whip is removed and the NMO capped with a cover. If it was more often it would be a problem, as I generally like to listen to broadcast FM and the whole swapping back and forth is annoying.

 

[Chorky]

Quotes edited for space savings

Your mounting sounds aesthetically pleasing.

As I am OCD astetics certainly takes part of the current idea of placement, but it is also a functional plan so that kayaks/canoes can be on a rack as well. Fenders are not out of the possibilities, but from everything I understand, highest and center of vehicle is optimum, so I had thought a 8' truck canopy top would be sufficient for spacing. But, maybe I am incorrect. Thank you for the mention about 'at least 1/4 wave apart'. This is something I was not aware of and helps a lot with planning! As for the canopy, yes I mean a shell, topper, so many different names people use. It is a steel frame, with aluminum skin (an older used gemtop workmaster), so there should be no problem with the ground plane. I certainly have considered as you said the idea of just swapping antennas around if performance is sub-par, and partly why I am trying to pre-plan. I currently have the GMRS and CB (not installed), but will be some time until I acquire a scanner, and my HAM - maybe this summer????? hah Hopeful. I do plan to start with a 2m/70cm with APRS support - tinkering is fun. So really want to try and pre-plan as best as possible to avoid future headache with holes...

The antennas whips themselves aren't really a concern but....

Thank you Dave for joining!! Ok so the mention from yourself and prerunner about damage. Say that, for sake of discussion, a GMRS and 70cm antenna were 2" apart. Would this potential damage only occur if both radios are on with one transmitting? Just so I understand better. Or would this potential damage occur regardless of the affected radio being on or off? My CB is a SSB and includes weather stations. Regardless of this, it does seem to me that maybe, to prevent possible damage, it would be preferable to consider placing one antenna (ham) toward the rear of the canopy, and the other (GMRS) on a front fender? However, correct me if I'm wrong, but would this in fact alter the transmit ability (signal shape) of the GMRS by being on the fender, and not being optimum such as on top of the roof/canopy?

And, as for the dual CB whip's. I do understand duals' causes issues. I was under the assumption from my info gathering that duals, despite their drawbacks, is still preferable for those who tow trailers. Maybe I am incorrect? For sake of further discussion, say I did move on with the duals, if one side was extended upright, and the other bent back so not to be exposed more than 6" over the canopy, would this essentially negate the negative effects, or make it worse? Or, if I did take your advice and use only one, would there not be significant signal drawbacks by having a whip on just one side of the canopy via ball mount, or does the sheer fact of it being 102" sort of void that possibility?

Still learning... but thank you for entertaining my questions!

 

[prerunner1982]

I know you didn't direct this towards me, but I am going to play anyways.

it would be preferable to consider placing one antenna (ham) toward the rear of the canopy, and the other (GMRS) on a front fender? However, correct me if I'm wrong, but would this in fact alter the transmit ability (signal shape) of the GMRS by being on the fender, and not being optimum such as on top of the roof/canopy?

I would leave it on top. The GMRS antenna is so short it would be blocked by the vehicle. The ham antenna may due a smidge better but in VHF/UHF antenna height is king. I noticed a difference on 2m when going from a vehicle with the antenna on top where I could just barely get into and hear a local but distant repeater to a vehicle with it on the fender where I couldn't hear or get into the repeater at all. I have also seen the difference when travelling with friends that had antennas on the roof that could talk over great distances where I could not.

And, as for the dual CB whip's. I do understand duals' causes issues. I was under the assumption from my info gathering that duals, despite their drawbacks, is still preferable for those who tow trailers. Maybe I am incorrect? For sake of further discussion, say I did move on with the duals, if one side was extended upright, and the other bent back so not to be exposed more than 6" over the canopy, would this essentially negate the negative effects, or make it worse? Or, if I did take your advice and use only one, would there not be significant signal drawbacks by having a whip on just one side of the canopy via ball mount, or does the sheer fact of it being 102" sort of void that possibility?

A single antenna on the side of the vehicle will cause some pattern distortion towards the direction with the most metal. If you mounted it on the side of the vehicle near the middle (front to rear) driverside your signal will be more towards the front passenger and the rear passenger. If you use a 102" whip you will need a 6" spring, however you can get a 108" whip where no spring is required.
Also on duals you need to have them 9' apart which is not easy to do.

Unless the trailer is significantly taller than the top of the truck it will have little to no impact on your antennas/radio performance.

 

[sonoronos]

Among other reasons, this is why filters are a thing. Notch filters might help.

 

[DaveInDenver]

Among other reasons, this is why filters are a thing. Notch filters might help.

^ The man is right you know. ^

The only place where I've been unable to really solve it is running a stand alone APRS radio. It's basically impossible to build a physically small enough filter with steep enough edges to filter an APRS radio at 144.390 against generally use on the rest of the band by another radio. It's got to be a large cavity filter, such as used with repeaters.

Otherwise a notch or bandpass placed strategically fixes a lot of issues.

 

[sonoronos]

One thing I think is important to point out with Dave's info on dual CB antennas is that he isn't saying that they cause negative issues.

Dave is simply addressing the practical difficulty of using dual CB antennas for the intent of increasing directivity. This is a two element antenna array, so first you have to separate the antennas so that they are not mutually coupling. Then, you have to consider the effect that feed-line phase and antenna spacing have on your resulting radiation pattern. The problem is that even with totally fixed radiators with ideal conditions, it's not easy to accomplish practically, especially in HF. Factoring in floppy whips and metal bodies everywhere makes it quite difficult to guarantee that you're going to get the intended directivity gain. The result is some unknown or variable response...which may not be better than having a simple single radiator.

Here is a PDF from UTEP discussing the situation:

http://emlab.utep.edu/ee4382_AntennaEngineering/Topic%206%20--%20Linear%20Antenna%20Arrays.pdf

 

[DaveInDenver]

As I am OCD astetics certainly takes part of the current idea of placement, but it is also a functional plan so that kayaks/canoes can be on a rack as well. Fenders are not out of the possibilities, but from everything I understand, highest and center of vehicle is optimum, so I had thought a 8' truck canopy top would be sufficient for spacing. But, maybe I am incorrect.

Above all the theory is that any antenna you mount has to be secure and not annoying to live with everyday. Right ********** dab in the middle of a roof is the best location but if it requires removing and installing every day to access a garage it'll grow old real quick. If a fender or trunk lip doesn't require that then just for your sanity it might be worthwhile to take the performance hit.

Not to mention you only have one center of the roof so that antenna must be your most important and all the others are going to necessarily be second fiddle. You might have two important radios but neither is flatly critical so you take a small hit with two by putting neither in the middle and off centering both to split the performance hit equally.

Thank you for the mention about 'at least 1/4 wave apart'. This is something I was not aware of and helps a lot with planning!

There's nothing magical about 1/4λ. The reactive near field is:



That's 5/8λ.

And the far field equation:



That's 4λ.

Between them is the radiating near field. You want ideally to be out in the far field, the signal attenuates quickly (exponentially in fact) when you radiate. But that's not really possible except at microwave frequencies on cars so IMO a better rule of thumb is to keep about 1λ separation on the longer/longest of your frequencies of concern. You'll see that this can really only be achieved with VHF and higher on a vehicle anyway, so even that can't be a hard-fast rule.

a GMRS and 70cm antenna were 2" apart. Would this potential damage only occur if both radios are on with one transmitting? Just so I understand better. Or would this potential damage occur regardless of the affected radio being on or off?

Two inches is extremely close. UHF in this case has a wavelength of about 27" and you're well within the reactive region where I think two antennas may interact readily. This IMO would be a rule you should follow, not to be unreasonably close (0.1λ). But in this case it's not just because it's another UHF antenna, a dinner fork, piece of trim, a roof rack tower, anything being that close could inadvertently react.

Another point to make here is also that antennas follow a principle called reciprocity, meaning if the characteristics of two whips create a quasi Yagi by proximity they will do so on both TX and RX. So it would actually perhaps be possible to change the received signal in your extreme example. But also realize that metal isn't always an antenna. Body parts aren't floating, they are grounded so are less likely to become reactive. Same with other radio antennas. They are terminated, either with a DC ground or at least into a radio so they aren't necessarily alway going to want to be come part of your antennas easily and may remain substantially invisible to RF.

Regardless of reciprocity though, damage is still only possible when you transmit. Whether the receiving radio needs to be powered on depends. There's more than one way to cause an issue.

A very large signal could damage a radio that is turned off but a designer can anticipate this by using protection in the front end. In that case it may be repairable and you might or might even notice (although the protection would presumably be lost). This is the sort of thing that could also result from static or a radio being protected against someone touching the antenna port to power. This would be like a fuse blowing, although it's actually a PIN diode or FET.

OTOH constantly hitting a receive circuit with a strong field (but not grossly so) may over time damage the radio by causing it to become less sensitive. This is like asking an engine to run at redline constantly. It might take a mile or 50,000 miles but you'll eventually find the stressed parts. In a low signal RF amplifier that may cause component values to drift over time, usually in a squelch or muting circuit, or an amplifier to slowly burn out. The radio just gets more and more deaf until you realize you're not hearing anything anymore.

It may also be repairable but this is not quite as straightforward being a component level troubleshooting.

Point here is if you're causing the radio to stop hearing distant stations when you transmit then you're swamping the front end, so it's not something you should ever do ideally. It's just a matter of something you may have to deal with with more than one radio in your truck. Doing like @Airmapper and remove antennas for radios you're not currently using is wise.

 

[PhulesAU]

Dual whips for 11 meters will get you Nothing. splitting 4 watts from a cheap cb is a losing propsition.

 

[Chorky]

This is all super interesting, and my curiosity is getting the best of me. I'll try and not stray too far though, for my own sanity

The GMRS antenna is so short it would be blocked by the vehicle.

Pardon my ignorance but what is considered 'short' in this term? I only ask because the GMRS antenna I currently have is a double 5/8, so it's about 35". When I got it, that's what someone convinced me would be optimum, although now I'm not so sure since I realize the performance in hilly terrain is reduced even though straight line distance is increased. Anywhoo, if on a fender, it would stick above the roof a little, but of course would alter radiation more to that corner and reduce behind the vehicle. I do plan still for it to be on top, but just curious what you mean by 'short' - I suppose to make sure I know what I think I know ha. I'm assuming your just talking about a standard 1/4 wave length antenna, which is shorter than mine and would not be above the roofline.

Among other reasons, this is why filters are a thing. Notch filters might help.

Filters, ok I need to look into this just to understand it better. Didn't think for mobile it would have been a 'thing'. But sounds like it may be a bit more involved than necessary too.

One thing I think is important to point out with Dave's info on dual CB antennas is that he isn't saying that they cause negative issues.
Dave is simply addressing the practical difficulty of using dual CB antennas for the intent of increasing directivity.

Right. So the original reason I was planning duals was due to the trailer situation, which I seem to have misunderstood its significance. The second being if a single antenna on one side reduces signal to the opposite side, then mounting one on either side would have slightly more evenness. And third honestly was ascetics. But maybe I need to rethink the duals. One other reason for having it side mounted (the mount would be on the canopy, which is about 4.5' from the ground - tall truck) is to easily pull it down and hook it on the roof rack so it's not sticking up in low clearance areas. Although, that doesn't help much with the other antennas center of the canopy - which likely will need to be pivot or spring mounted if its a thing as I haven't seen that sort of mounting option yet. That PDF was a great read and caused google to be my friend for a while. A bit over my head at the moment but I get the main points, in the sense that duals may be more trouble than theyre worth. The original idea behind them was not to purposefully increase forward/rearward distance, but rather to not have a 'blind spot' on the side opposite of a single mounted whip. Maybe I was making a bigger deal out of it than it really is?

Above all the theory is that any antenna you mount has to be secure and not annoying to live with everyday. Right ********** dab in the middle of a roof is the best location but if it requires removing and installing every day to access a garage it'll grow old real quick. If a fender or trunk lip doesn't require that then just for your sanity it might be worthwhile to take the performance hit.
Not to mention you only have one center of the roof so that antenna must be your most important and all the others are going to necessarily be second fiddle. You might have two important radios but neither is flatly critical so you take a small hit with two by putting neither in the middle and off centering both to split the performance hit equally.
...
Point here is if you're causing the radio to stop hearing distant stations when you transmit then you're swamping the front end, so it's not something you should ever do ideally. It's just a matter of something you may have to deal with with more than one radio in your truck. Doing like @Airmapper and remove antennas for radios you're not currently using is wise.

Thank you for clarifying, that helped quite a bit. So what I understand from this basically is that although important, not necessarily much that can be done other than dealing with the compromise, and making sure you dont' cause damage. Possibly, considering my original idea in the first post, would be changing the mounting so that the GMRS antenna is toward the front of the canopy, while the 2m/70cm is toward the back, giving a more healthy 6' between the two, then the other antennas, such as the cell extender, can go between them. Although this would require the scanner antenna to be somewhere else entirely. I suppose one could be center of the truck cab via a mag mount, but I'm really not a fan of those since wires just lie loosely on the roof then. As you mentioned about not being annoying, that's going to be difficult regardless since I'm relatively short, and the truck is relatively tall, which requires a ladder for me to reach the top to either remove or (if spring/pivot mounted) lower the antennas in areas of thick brush or low clearance areas. This is another solution to a problem I have not yet figured out...

Dual whips for 11 meters will get you Nothing. splitting 4 watts from a cheap cb is a losing propsition.

Interesting, ok. I honestly did not consider this. I have the President McKinley SSB, and the Midland MXT 400. scanner and 2m/70cm will come later when I decide which to get, and get my license.

 

[prerunner1982]

Pardon my ignorance but what is considered 'short' in this term? I only ask because the GMRS antenna I currently have is a double 5/8, so it's about 35". When I got it, that's what someone convinced me would be optimum, although now I'm not so sure since I realize the performance in hilly terrain is reduced even though straight line distance is increased. Anywhoo, if on a fender, it would stick above the roof a little, but of course would alter radiation more to that corner and reduce behind the vehicle. I do plan still for it to be on top, but just curious what you mean by 'short' - I suppose to make sure I know what I think I know ha. I'm assuming your just talking about a standard 1/4 wave length antenna, which is shorter than mine and would not be above the roofline.

I guess I should have asked what antenna you were using before I opened my mouth. Figured it was a 1/4 (about 6") or 1/2 wave. A 5/8 over 5/8 wave is plenty long, and higher gain. It would probably work ok on the fender, but not as well as on the roof.

 

[Chorky]

I guess I should have asked what antenna you were using before I opened my mouth. Figured it was a 1/4 (about 6") or 1/2 wave. A 5/8 over 5/8 wave is plenty long, and higher gain. It would probably work ok on the fender, but not as well as on the roof.

No worries, I figured you were talking about a 1/4 wave, but just wanted to verify . And by my own mistake and fault, I did not do enough research when I got the antenna. Which was a year or so ago. every time I think I nail down what is the best set of compromises (for me), I learn something new haha, which is why the radios are still not even in the truck. Now I'm reconsidering not using this antenna and going to a 1/2 wave, or the 'standard' 1/4 wave. After understanding better what the pros/cons and impacts are relating to gain (although I don't quite understand a double 5/8 fully just yet, but getting there - I was talked into it), I'm not sure a high gain antenna is optimum, for me anyway.... Distance is great, but at the sacrifice of obstacles, like hills/mountains. I'm more frequently in mountainous/hilly terrain than not. (currently in western wa, soon moving to western mt). But regardless I still think that roof top mounted is optimum even with a double 5/8 antenna. Its just a matter of correct spacing between the future planned 2m/70cm. Anyway, back on topic