Antenna separation question.
- Thread starter kidphc
- Start date
prerunner1982
Adventurer
I have one 5/8 wave 2m antenna in the middle (more or less) of the roof of my XJ and another on the passenger rear corner of the roof. Maybe 2.5-3' apart and rarely have much of an issue. The one in the middle is for voice and the other is for APRS. I also have a 102" whip with an adjustable loading coil on the driver's rear corner for HF and no problem there either. The further apart the better, but work with the room that you have... you don't want them to touch though, mine are probably a bit close given the length of the antennas.
kidphc
Member
Thanks. Helps me alot. Since I wanted dual vhf/uhf rigs on the roof and maybe a no ground plane Cb on the front fender.
DaveInDenver
Middle Income Semi-Redneck
More is better and it's likely if you're less than even a few wavelengths the receiving radio will be overloaded and unable to hear anything while the other is transmitting. That's called desense, short for desensitizing the radio. Where exactly this goes from annoying to damaging isn't cut and dry.
If your radio is exposed to a very strong local field long enough it'll probably ruin it eventually. Key part is "eventually". Very close (like an inch) that might be a handful of exposures even at low power. Go out to a foot, 5 feet, 15 feet the time it takes goes up exponentially until it's essentially never going to damage anything even at 50 or 100 watts.
So my rule of thumb (which is worth a cup of coffee at best) is (1) NEVER let antenna whips touch and (b) try to stay one wavelength apart on the lowest band of interest (e.g. 2 meters apart if you're looking at two 2m ham radios).
Rule (b) is subjective because it's impossible below 2m to achieve this on a vehicle and even 2m mobile is a tough one. So just do your best and remember the rule to use the lowest power you can.
Rule (1) should never be ignored, especially with metal whips. If you short a transmitting radio antenna to another radio you'll almost certainly ruin one or both quickly. Maybe not once or twice but it's not something you want to do very often.
BTW, should mention that when dealing with 2m VHF you should also consider it's location relative to your vehicle commercial AM/FM radio. At 50W a 2m ham radio can upset 88-108 FM radios pretty badly. Also think about GPS antennas and devices and things like Sirius/XM or InReach/SPOT. Those devices operate in a much higher range of frequencies but they have extremely sensitive RF front ends that could be easily damaged, not to mention practical issues like throwing off your navigation. They need a clear line of sight to the sky free of interference and a couple of feet of separation can't hurt.
If your radio is exposed to a very strong local field long enough it'll probably ruin it eventually. Key part is "eventually". Very close (like an inch) that might be a handful of exposures even at low power. Go out to a foot, 5 feet, 15 feet the time it takes goes up exponentially until it's essentially never going to damage anything even at 50 or 100 watts.
So my rule of thumb (which is worth a cup of coffee at best) is (1) NEVER let antenna whips touch and (b) try to stay one wavelength apart on the lowest band of interest (e.g. 2 meters apart if you're looking at two 2m ham radios).
Rule (b) is subjective because it's impossible below 2m to achieve this on a vehicle and even 2m mobile is a tough one. So just do your best and remember the rule to use the lowest power you can.
Rule (1) should never be ignored, especially with metal whips. If you short a transmitting radio antenna to another radio you'll almost certainly ruin one or both quickly. Maybe not once or twice but it's not something you want to do very often.
BTW, should mention that when dealing with 2m VHF you should also consider it's location relative to your vehicle commercial AM/FM radio. At 50W a 2m ham radio can upset 88-108 FM radios pretty badly. Also think about GPS antennas and devices and things like Sirius/XM or InReach/SPOT. Those devices operate in a much higher range of frequencies but they have extremely sensitive RF front ends that could be easily damaged, not to mention practical issues like throwing off your navigation. They need a clear line of sight to the sky free of interference and a couple of feet of separation can't hurt.
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kidphc
Member
Much appreciated. Probably only going to be able to get at max about 20-24 inches of separation.
Desense really what i was worried about. Have a radio go deaf and get damaged. Hurts when both radios I was looking at are about $400 each.
As far as the antennas not touching going to be tough when 1/2 wave 2m antennas are about 3 feet in length.
Why Kenwood can't you already just build a mobile version of the Th-d74a. Half of the mobile ham community is waiting. You know touch screen, dstar, full tnc aprs, with a repeater database via gps.... that thing would fly off the shelf as long as it was not $1000.
Sent from my SAMSUNG-SM-G891A using Tapatalk
Desense really what i was worried about. Have a radio go deaf and get damaged. Hurts when both radios I was looking at are about $400 each.
As far as the antennas not touching going to be tough when 1/2 wave 2m antennas are about 3 feet in length.
Why Kenwood can't you already just build a mobile version of the Th-d74a. Half of the mobile ham community is waiting. You know touch screen, dstar, full tnc aprs, with a repeater database via gps.... that thing would fly off the shelf as long as it was not $1000.
Sent from my SAMSUNG-SM-G891A using Tapatalk
kidphc
Member
Should mention the am/fm radio antenna is right fender right by the glass. The 2/70sh is going to be mounted via nmo in the center of the truck. The second antenna was going to be mounted 20 or so inches behind. First, was for situational coms and aprs. Secondary, antenna which was going to be used for Dstar.
Gives each antenna around 20" of ground plane.
So far have the Ftm400 on order since Aprs and some coms where prioritized.
Sent from my SAMSUNG-SM-G891A using Tapatalk
Gives each antenna around 20" of ground plane.
So far have the Ftm400 on order since Aprs and some coms where prioritized.
Sent from my SAMSUNG-SM-G891A using Tapatalk
DaveInDenver
Middle Income Semi-Redneck
LOL, I think Kenwood should build a TM-D710 with DMR. That is something I would buy.
dreadlocks
Well-known member
amen to that, I'd buy a couple of em more than likely.. touchscreen aint all that great.. terrible to use while driving, cant feel any buttons gotta take your eye off the road.. then when you reach out to push something and hit a bump you push something else and gotta take your eyes off even more to figure out what you pressed.. all it does is make it seem less intimidating without all the physical buttons.. on a handheld its cool though.. because I dont want a million buttons in my pocket or hanging off my belt getting pressed inadvertently.. in my car though I can operate my D710 blindfolded if I had to.
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DaveInDenver
Middle Income Semi-Redneck
That's interesting in discussion and presentation of formulas but it's kind of not useful here. Their conclusions are based on the same band aggressor radio having no affect on the victim so they find distances in kilometers. It seems geared to how to identify suitable repeater sites for shared/trunked systems like for public service for ideal performance.
It does support that there is really no safe distance on a car for two antennas unless you put in protective circuits, e.g. duplexers and diplexers or band pass/reject filers to get you the equivalent attenuation that distance does.
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