Antenna spacing

[KJ4OBR]

Stupid questions alert! LOL

 If I'm using an HT with 5w output feeding an external 1/4 antenna for my APRS packets, how close is too close to mount the APRS antenna in relation to my mobile antenna. Testing with a mag mount (not the final solution) is yielding brief desense in the RX of the mobile (VHF and UHF) when the APRS packet is sent even when the antennas are as far apart as possible while remaining on the car. Backing off on the power of the APRS radio doesn't get the packets heard.

Question 2 how bad is it for the main radio to desense during the brief APRS burst?

Thanks n 73

[K5LXP]

Do you mean "bad" as in damage?  I wouldn't worry about it.  If you maintain a minimum of a quarter wavelength of separation you'll be fine.  There's not much you can do for the desense, so get used to the voice radio dropping out when the APRS keys.


Mark K5LXP
Albuquerque, NM

[AB4D]

The desense caused by your APRS rig is probably nothing more than an annoyance, than anything that would cause damage. It should not damage the other radio at the power level you indicated.

However, you should ensure that you mount the antennas far enough apart in a manner they could never come into contact. If they were to touch while transmitting, it could damage your radio system by inducing too much RF into the front end of the receiving radio.

Always think ahead if you are punching a hole in your vehicle for an antenna mount.  Will you always use these mounts for that purpose?  Keep in mind your current uses, and your future uses as well.

73, Jim.

[K7RBW]

You could always fill your trunk with a few tuned cavities to notch out the APRS frequency from your other transceiver.  You'd need to be careful not to ever try to transmit on the APRS frequency from the non-APRS radio (because the antenna would be shorted to ground at that frequency), but aside from that, 3 or 4 tuned cavities should reduce the APRS signals on your transceiver by 80-120 dB.

But, I think living with the occasional (and brief) desense is probably the cheaper and more practical option.

[WB6BYU]

For a standard mobile rig, I get desense at about 2 car lengths away (measured when stopped at a
traffic light.)  This will vary somewhat depending on the radio design and the transmitter power,
but you won't be able to separate the antennas enough to prevent it.  It can be annoying, but not
likely to damage the radios unless they are very close together.  (The main radio will suffer desense
from the APRS transmitter as well.)

If you want to be safe, place them as far apart as is reasonable (perhaps one on the roof, one on
the trunk lid.)  But you regularly see mobile "porcupines" with lots of antennas that don't damage
the other radios.  (Our Search and Rescue van had at least 9 antennas on the roof, and one
transmitter ran 100 watts.)

[KJ4OBR]

Much as the idea of getting to play with tuned cavities is appealing I think I'll just live with the blips LOL.

Thanks all for backing up my gut feeling.

73

Dave

You could always fill your trunk with a few tuned cavities to notch out the APRS frequency from your other transceiver.  You'd need to be careful not to ever try to transmit on the APRS frequency from the non-APRS radio (because the antenna would be shorted to ground at that frequency), but aside from that, 3 or 4 tuned cavities should reduce the APRS signals on your transceiver by 80-120 dB.

But, I think living with the occasional (and brief) desense is probably the cheaper and more practical option.

[AA4HA]

Since we are talking about whip antennas as usually installed on a vehicle they are also vertically polarized. If you were able to stack the antennas (on a different horizontal plane, separated by around a wavelength) you would get pretty good isolation from antenna to antenna.

Putting verticals on a flat surface; bonnet or roof of a vehicle, building top, etc... Places all of the other antennas directly in the main radiation lobe of the other vertical antennas. On building rooftops I have seen signal levels of +40 dBm (10 watts) on a receiver antenna from a nearby 500 watt vertical antenna that was operating several MHz away (928 MHz pager transmitter hitting a 932 MHz MAS point-multipoint repeater input). The desense was quite significant as the repeater would go into major AGC action, making it deaf to those sites out there that were trying to come in at -90 to -110 dBm. Weak sites would work intermittently (between gaps in the pager transmitter operation) but stronger sites could still capture the receiver.

There was a triple cavity filter on the repeater input but it did not have a high enough Q to attenuate the passband signal level low enough to prevent the desense. I ended up adding an 8 pole programmable pass filter;
  ( http://www.qfilterproducts.com/ultra-q.html )
I was able to knock down signals that were as close away as 25 KHz to the point where I did not have desense on the repeater. It also eliminated an intermod problem as the repeater was going non-linear from the high signal level.

If it is possible, I never place vertically polarized antennas that are anywhere near each other (frequency-wise) on the same horizontal plane. When spacing vertically polarized antennas in a stacked configuration on a tower or mast I always go at least a wavelength apart (minimum 3', irrespective of frequency, even up into the microwave bands).

For horizontally polarized antennas the problem will be exactly the opposite.

Remember too that whenever you end up in a complex antenna environment you can have the other antenna elements acting in a passive manner, creating "directors or reflectors" and your omni pattern can be significantly altered. If you do NEC modeling (wire models of vehicles and the antennas) then you need to represent all of the other antennas. You will really be surprised by what that does to your expectations of how an antenna should behave.