scanner antenna next to 75w APRS antenna causing harm?

newsphotog · #1 (**permalink**) 05-21-2009, 04:06 PM

Wirelessly posted (BlackBerry8330/4.3.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/105)

I am using a Larsen tri-band 140/440/800 antenna mounted on the trunk lid connected to a Uniden BCD996T.

And then about 10 inches away on the trunk lip, I have a Diamond NR72BNMO dual-band antenna which is connected to an Icom V8000 that I use for APRS. Only when I am in an area that doesn't have very good APRS coverage, I run the Icom at 75 watts. The rest of the time I run it at its "low-mid" power setting, whatever that might be, I'm not sure of the wattage. The TinyTrak is set to beacon every 3 minutes.

I previously had a BCT15 as a scanner, but suddenly it wasn't getting very good reception. My installer got it and tested it compared to the BCD996T and found there was certainly something wrong with the BCT15. I put the BCD996T in the car and then sent the BCT15 to Uniden for repair. I checked the status of the repair and they are just now starting to look at it and it should be shipped back out in a little over a week.

Now, my BCD996T is having a similar prblem. It is having reception troubles. I cannot receive any VHF/UHF, although 800MHz reception isn't suffering as much -- I still get some signals but not as much as I should -- but I still can't receive anything on 140 or 440.

I am pretty sure it's not the antenna, because it worked when I swapped scanners.

My question is this: if I let the APRS radio transmit at 75 watts at about 10 inches away from the scanner antenna, could the RF from the APRS radio go through the scanner antenna and into the scanner and cause some damage?

DDan · #2 (**permalink**) 05-21-2009, 04:41 PM

Absolutely and this isn't the first time I have heard of this. At 75 watts I would disconnect the scanner antenna while transmitting.

gewecke · #3 (**permalink**) 05-21-2009, 04:47 PM

Try reducing the power output of your icom?
N9ZAS

newsphotog · #4 (**permalink**) 05-21-2009, 09:17 PM

Quote:

Originally Posted by DDan

Absolutely and this isn't the first time I have heard of this. At 75 watts I would disconnect the scanner antenna while transmitting.

Wow, that really sucks. I bricked two scanners. After talking to an old ham, he told me that when APRS first started out it wasn't uncommon for these things to happen. Which is weird, since I had asked him before if this was a bad idea and he said no...

Would 50 watts be too much? I thought a lot of people sometimes run 50w in the boonies for APRS.

Quote:

Originally Posted by gewecke

Try reducing the power output of your icom?
N9ZAS

Yeah, obviously, I turned it down today but the damage has already been done.

rvictor · #5 (**permalink**) 05-21-2009, 09:35 PM

Quote:

Originally Posted by newsphotog

Wow, that really sucks. I bricked two scanners. After talking to an old ham, he told me that when APRS first started out it wasn't uncommon for these things to happen. Which is weird, since I had asked him before if this was a bad idea and he said no...

Would 50 watts be too much? I thought a lot of people sometimes run 50w in the boonies for APRS.

Yeah, obviously, I turned it down today but the damage has already been done.

I don't know that anyone can tell you the exact power level that would be safe, but I wouldn't put 50 watts through an antenna 10 inches from my scanner antenna. If it were me, I'd move one of those antennas as far from the other as I could.

I'm not absolutely sure, but I somewhat recall that even if the scanner (or other receiver) is turned off it can suffer damage from excessive input signals.

I suppose that some kind of antenna switching relay could be used to disconnect the receive antenna prior to transmitting on the other antenna, but that seems like a considerable project for a transmitter that transmits automatically every few minutes. If it were strictly a voice transmitter, you could manually switch the receive antenna off prior to transmitting, but then there's always the possibility of forgetting. I'd move one of the antennas.

73
Dick

N_Jay · #6 (**permalink**) 05-21-2009, 10:05 PM

If you want to make a difference, you probably need to get the signal into the scanner 6 to 10 dB lower.

1.75 dB is not significant.

Get more space between the antennas

w8jjr · #7 (**permalink**) 05-22-2009, 01:55 AM

Oh yea even 5 watts will at times fry the front end of the scanner.
not sure what you drive but you need to place them on roof and trunk to be safe.

newsphotog · #8 (**permalink**) 05-23-2009, 03:06 PM

I drive an Impala, and my scanner antenna is too tall for my tastes to be mounting on the hood (I like to have a little bit of stealth). So far I think my options are to mount the scanner antenna on one of the front fenders, use a ~20dB attenuator or find a 2m notch filter (preferred option). Where can I find one?

I'm not really willing to part with the scanner, ham radios, or APRS so I have got to make this work.

The first scanner I fried was shipped back to me from Uniden yesterday, it should be here on the 29th. According to Uniden's repair status website, it doesn't really say what the problem was, but it said they replaced parts D1, D13, D14, D2, D4, D5.

mrweather · #9 (**permalink**) 05-24-2009, 07:55 AM

Sounds like a bunch of diodes got fried. Keep in mind radio front ends are used to dealing with signal levels in the microvolt range. Injecting 75 watts of RF from 10 inches away most likely sent three, four or more orders of magnitude of voltage down the line. Not good.

There are commercially available notch filters but I don't know if they would knock down the RF sufficiently.

Repeater duplexers would certainly work but then your trunk would be full of cans!

phatboy48 · #10 (**permalink**) 05-24-2009, 08:29 AM

I spent nearly 40 years as a RF communications technician and have seen this sort of problem before. You need to get at least five or more feet between those antennas if you are going to run 75 watts at VHF frequencies. The scanner front end circuitry won't be able to withstand more than a few tens of milliwatts of incident power being applied before bad things happen.

If you are unable to physically separate your antennas, then you should contact somebody like PAR electronics about a custom notch filter for the antenna of your scanner tuned to reject the APRS frequencies you are using. If you can get around a 30 dB notch, that should be adequate, especially if you can get a little more separation between your antennas.

73....John
W8JNC

newsphotog · #11 (**permalink**) 05-24-2009, 03:58 PM

A buddy found a notch filter made by Stridsberg that is about -20 dB at 150 MHz. It's the FLT225 (Receive Filters, FM Notch and High Pass models). I'll lose a lot at 225 MHz and below, but most of those frequencies are rural agencies that I rarely need (although I'm sure after I install the filter, something big will happen and I'll need those frequencies again). I haven't been able to find a better filter than that one, unless I order a custom-made filter. It wouldn't hurt to use a 2-meter filter on my scanner since I also use an ID-800H about 10 inches away from the scanner as well, though I rarely run that at it's full power, 50w.

I don't think I will be running APRS at 75w anymore, I am thinking about replacing the IC-V8000 with a Kenwood D710A. Either way, I am considering using an Antenex ETRAB1440 phantom "shark fin" antenna. I will probably install this on the roof, either directly in the center or as forward as possible near the windshield. However the max power rating on that antenna looks like 60w, but if I am using a TM-D710A that won't be a problem.

Any thoughts?

phatboy48 · #12 (**permalink**) 05-24-2009, 09:13 PM

Another time proven performer is the old quarter wave stub. The idea here is to insert a tee connector in line with the antenna cable to the scanner. Next, cut a piece of rg-58 cable 20 inches long and install a connector on one end while leaving the other end open. Connect this stub onto the open port of the tee connector. Now comes the tricky part. You need to tune the scanner to a weak signal either on or very near to the frequencies in question. Any ham repeater on the 145 mHz segment would probably suffice. Next, while listening to the weak signal, take a diagonal cutter and begin clipping very short pieces from the end of the cable, no more than 1/8" max. After clipping a few pieces, you should notice the signal beginning to get weaker with each piece you clip off. You should find a spot where the signal doesn't get any weaker and will start to strengthen again if you continue cutting. Stop here!

This stub filter will have about 20-25dB of signal rejection at the tuned frequency. It will have minimal loss up in the 154-170 mHz band. NOTE...due to the impedance repeating characteristics of coaxial cable, this filter will also have similar loss at 3X the tuned frequency where it acts as a 3/4 wavelength stub. That should not have a serious effect at the UHF public safety channels.

You can easily make this stub filter tunable by cutting an additional 1/4 inch or so off beyond the above mentioned "stop point" and then connecting a small trimmer capacitor across the open end of the cable to tune the notch frequency. Something in the neighborhood of 5-15 pF should do nicely.

John

N1BHH · #13 (**permalink**) 05-25-2009, 12:00 PM

It is always best to have antennas spread as far as practicable from each other. That scanner front end is getting swamped to the point it will become deaf and unusable. But that's only makes common sense.

zz0468 · #14 (**permalink**) 05-25-2009, 01:19 PM

An Impala? I'm sure there's enough real estate on a vehicle that size to get sufficient separation without having to resort to notch filters and other such nonsense. I've always found that mounting one antenna on the roof, and one on the trunk lid worked just fine to keep radios apart, and that was with 100 watt class radios.

Forget the "stealth" part of the equation. You lost that with the first antenna...

newsphotog · #15 (**permalink**) 05-28-2009, 01:26 AM

Quote:

Originally Posted by zz0468

An Impala? I'm sure there's enough real estate on a vehicle that size to get sufficient separation without having to resort to notch filters and other such nonsense. I've always found that mounting one antenna on the roof, and one on the trunk lid worked just fine to keep radios apart, and that was with 100 watt class radios.

Forget the "stealth" part of the equation. You lost that with the first antenna...

I think I will end up mounting most antennas on the roof with a shark fin-type antenna from Laird/Antenex and then leaving the scanner antenna on the trunk. Gonna drill them holes too!

burner50 · #16 (**permalink**) 05-28-2009, 02:02 AM

50 watts is plenty...

I know of people who have traveled thru fort dodge at 50 watts and had no problem with APRS

odontia32m · #17 (**permalink**) 05-28-2009, 07:37 PM

I killed my pro-197 today by having my scanner antenna 12 inches from my 2 meter antenna, both NMOs.

? How far is far enough away?

? Can it hurt my scanner even when off?

BTW I have written GRECOM for their engineers answer. It is not totally dead but wounded in action.

I have a chevy pick up. Both are now in the roof.

odontia32m · #18 (**permalink**) 05-28-2009, 08:06 PM

Another question. How do cops not kill their front ends? Some of the state boys have 7 antenna real close?

jim202 · #19 (**permalink**) 05-28-2009, 08:50 PM

Over the years I too have managed to blow the front end of some commercial two way radios. Being
in the repair field, I chose to take an active stand and try to prevent this from happening again. my
solution was to add a pair of hot carrier diodes back to back across the front end of the receivers.
If there was no series resistor coming off the first tuning coil to the active front end, I would add a
small value resistor to provide a means to drop a voltage when the diodes did conduct. After the
mod, I never had a blown front end again.

One of my biggest sources of RF was from a high powered UHF radar at the facility I once worked
at. It only ran about a megawatt of output power. The road ran about 30 feet away from the big dish
antenna that it used. it cleaned out the front end of my VHF radio twice before I figured out the
cause and modified the receiver.

Now tell me your little 75 watts couldn't be the cause.

Jim

odontia32m · #20 (**permalink**) 05-28-2009, 09:22 PM

Quote:

Originally Posted by jim202

Over the years I too have managed to blow the front end of some commercial two way radios. Being
in the repair field, I chose to take an active stand and try to prevent this from happening again. my
solution was to add a pair of hot carrier diodes back to back across the front end of the receivers.
If there was no series resistor coming off the first tuning coil to the active front end, I would add a
small value resistor to provide a means to drop a voltage when the diodes did conduct. After the
mod, I never had a blown front end again.

One of my biggest sources of RF was from a high powered UHF radar at the facility I once worked
at. It only ran about a megawatt of output power. The road ran about 30 feet away from the big dish
antenna that it used. it cleaned out the front end of my VHF radio twice before I figured out the
cause and modified the receiver.

Now tell me your little 75 watts couldn't be the cause.

Jim

I wish I knew how to do that.