Mobile Radios / Scanners / NMO antenna’s and RF feedback.

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fireboat61

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I am continually learning about Radios / power and antenna’s and RF feedback, but I know I have much more to learn.

Here’s my set up

1 - CS800D Mobile @50 Watts with a browning NMO antenna for VHF.

1 - Home Patrol 2 with an all band Browning antenna design for Multiband apx mobiles.

Both antenna’s are nmo mount on my trunk of my Ford Fusion. I measure exactly 19 inches from antenna tip to antenna tip. This is where I maybe confused.

I recently developed problems with my HP2 and lost the ability to receive anything. I believe I may have transmitted to much RF power into my scanner. Now I have to ship it out to fix it.

Questions:

1. Did I cause this HP2 problem ?

2. I read about the 19inch distance, is this information in correct ?

3. Are there any products I can add inline to prevent to much RF power from going into my scanners , without reducing there receiving capabilities ?

I want to understand more and prevent issues before I build my mobile set up in my truck.

I also recently purchased the Uniden SDS100 and definitely do not wont to blow it up. I am afraid to use my MD2017 on Low power In the same cab as the scanner. I can’t afford to blow the SDS100 up.

Any help would be appreciated. I am a young Ham with a background in public safety not electrical or ham related activities. I am excited to learn but not through destroying scanners or radios.

Thank you

KC3IQK


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fireboat61

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Here’s my antenna’s on my car.
e5543db123fce8d6a0747001e21d81f3.jpg



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ka3aaa

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Put the antenna for the scanner on the hood or use the oem am/fm antenna for the scanner. Space between antennas is your friend and its better to be safe than sorry. 50 watts blew the front end out of the scanner thats why its not recieving.
 

hill

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Antennas are way to close. Just because 19" is 1/4 wavelength on 2 meters they must still be farther away to protect the scanner.
 
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mmckenna

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1. Did I cause this HP2 problem ?

Probably. Too much RF from the transmitting radio getting into the receiver of the scanner and it'll wipe it out.

2. I read about the 19inch distance, is this information in correct ?

It's a good place to start, but it's not a hard and fast rule. There's some science involved (it's 1/4 wavelength on the 2 meter amateur band), but it also depends on antenna gain, how much power you are running, how susceptible the receiver is to high RF, etc.
Scanners usually have very wide filtering, so there's not really much to stop the energy once it's there.


3. Are there any products I can add inline to prevent to much RF power from going into my scanners , without reducing there receiving capabilities ?

No. You need more separation between the antennas.

I want to understand more and prevent issues before I build my mobile set up in my truck.

I also recently purchased the Uniden SDS100 and definitely do not wont to blow it up. I am afraid to use my MD2017 on Low power In the same cab as the scanner. I can’t afford to blow the SDS100 up.

Any help would be appreciated. I am a young Ham with a background in public safety not electrical or ham related activities. I am excited to learn but not through destroying scanners or radios.

Antenna separation is key.
I'm running 50 watt VHF commercial radios with about 22" of separation between it's 1/4 wave whip and the 800MHz antenna on my work truck. Both radios are transceivers. Both radios have filtering designed to reject most out of band stuff. That works to my advantage.

The scanners, having really wide, or no, filtering would be a bigger issue if I was running one.

Even with proper separation, you are going to get some amount of RF into the scanner. Often it will desense the receiver, in other words, the strong transmitted signal will temporarily deafen the scanner. When you stop transmitting, the issue goes away.
The trick is knowing how much RF your scanner can handle before it goes "poof". And there's no way to really know that, other than we can safely say that you found it.
You can hook up a watt meter to the scanner antenna and transmit with your 2 meter radio and see how much energy is showing up. That can help you decide on an antenna location.


As for the 19" spacing, that works well when it's two transceivers with good filtering, not so much with scanners. You'd really want more.

However, remove the scanner from your car and test it with another known good antenna and see if it works. Don't rule out wiring issues, bad connectors, bad antenna, etc.
 

FKimble

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You said you measured tip to tip = 19 inches. One is much taller which would actually make the two antenna much closer than 19 inches.

Frank
 

prcguy

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There is nothing magic about 19 inch spacing to reduce interference except that its more than 18 inches and less than 20 inches. All are a bit too close for a transmitter and receiver within the same band.

If you only use the CS800D on 2m amateur, you could get a VHF notch filter for the scanner that removes 2m from the scanner antenna input. Depending on the notch filter you would loose some reception either side of the 2m amateur band but it may not affect what you scan.
 

jim202

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Years back I was running a GE Delta SX in my truck when I worked at an MIT research site up in MA. I found that the receiver went deaf one day. Had to replace the front end FET in the radio. About a month later it went deaf again.

Thought about it this time. I drive by a 440 MHz. radar 150 ft. antenna every day getting to the building I worked at. That radar was running a whole bunch of power. Decided to add a couple of back to back hot carrier diodes from ground to the input coil output, before the coupling cap that fed the FET in the front end of the receiver.

After that I never blew the front end FET again. Not saying this is the solution here, but it is something to consider. It is an easy fix and you don't have to worry again about high level RF killing your receiver.

Jim
 

fireboat61

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Thank you guys for all the information. I have learned some new things today. It sucks to learn the hard way but it’s still learning. I will look into which options I will take, whether relocating the antenna forward or just putting the 2 meter RF block or just set back and take it from scratch. I appreciate the help and look forward to hopefully more members advise.


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fireboat61

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One more question, if I transmit from my
MD2017 on high power (5 Watts ) is that going to be an issue with my Uniden SDS100. I have my SDS100 secured with a mic clip to my dash about 2 - 3 ft from when I was transmitting using DMR. Im use the SDS100’s standard antenna which means the entire unit is in the cab.


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