RF Limiter Use/Operation

[tweiss3]

So when I last updated the equipment in the family vehicle, I ended up with scanner antenna in an existing drilled position in the back with a new mount/cable and Aligient 5086-7261 RF limiter, while I installed a new mount for the radio about 4-5' forward of the scanner, in the dead middle of the roof per the Ford up fitter locations.

My question is how exactly does the RF limiter work? Does it attenuate the signal? Does it bleed off excessive RF? Is it like a fuse and pops when RF levels are exceeded?

I only ask because the wife wants to put a roof rack cargo shell on for a few trips, and I don't want to loose VHF, but I'm limited on my options. I don't want to put on a 1/2 wave VHF whip, but any location open would have an odd pattern if I used a 1/4 whip. Obviously the "ideal" location is a lip mount on the rear hatch, but that would be about 2' from the scanner antenna.

 

[900mhz]

are you going to use any radio equipment that transmits in that vehicle? A limiter does what its name implies...it limits the amount of RF which may back feed through the antenna back towards the scanner, to prevent front end overload, and possibly blowing out the front end. Be advised limiters have a limit as well regarding the total amount of RF power it can handle on the input port.

 

[prcguy]

An RF limiter is usually two or more diodes across a transmission line and they could be a combination of silicon or Schottky and Zener depending on what RF level its designed to clamp. The first thing you should do when considering one is actually measure what RF levels you are dealing with so you can get one rated far above what your receiver may encounter.

The common HP/Agilent SMA versions can handle up to 10 watts briefly and they will start conducting somewhere around -10dBm to 0dBm and will usually limit the output to around 10dBm or 10 milliwatts which should be a safe level for any receiver.

I had one from DX Engineering rated up to 10 watts for HF use and I ended up blowing that one up due to too much coupling between antennas and too much power. I also picked up a couple rated 60 watts or so and now use that.

 

[tweiss3]

The one I have was listed as 10W. Considering the antennas are at least 4' apart, it was just a quick factor of safety so I didn't blow the front end on the SDS200. Transmit power is only ever going to be 50W maximum.

If I use a power density calculator, 50W into a no-gain antenna (2.2 dbi) with 2' spacing provides 1.77 mW/cm2. Being this is significantly below 10W, I should be ok? Am I looking at this correctly?

 

[prcguy]

At 2ft spacing on VHF, the two antennas are probably within the near field and its very difficult to calculate the coupling. Within the nearfield levels can vary quite a bit but I don't think the coupling would be less than 10dB or 5W at the receive antenna using a 50 watt transmitter.

The one I have was listed as 10W. Considering the antennas are at least 4' apart, it was just a quick factor of safety so I didn't blow the front end on the SDS200. Transmit power is only ever going to be 50W maximum.

If I use a power density calculator, 50W into a no-gain antenna (2.2 dbi) with 2' spacing provides 1.77 mW/cm2. Being this is significantly below 10W, I should be ok? Am I looking at this correctly?

 

[KevinC]

Attach a good quality watt meter to the RX antenna coax with a dummy load on the other end of the meter. Key up the transmitter and see what you read.

 

[tweiss3]

At 2ft spacing on VHF, the two antennas are probably within the near field and its very difficult to calculate the coupling. Within the nearfield levels can vary quite a bit but I don't think the coupling would be less than 10dB or 5W at the receive antenna using a 50 watt transmitter.

I'm going to have to pull out a tape measure and verify the exact locations I think things can go.

Attach a good quality watt meter to the RX antenna coax with a dummy load on the other end of the meter. Key up the transmitter and see what you read.

I don't have all the materials for this change (and the SDS200 is tucked away nicely, making pulling it out to test that SMA connector pretty difficult) so I can't test it. I'm trying to do the front end planning and ruling out what won't work before I spend more money.

Using this Calculator Friis Transmission Equation Calculator
And the antenna information
Scanner NMO150/450/758: 2m-0.0dbi, 70cm-5dbi
Compactenna 2/440+: 5dbi
NMO 2/70b 2m-2.4dbi, 70cm-4dbi

At 3' separation:
2/440+ to scanner is 4W/1.6W (2 meters/70 cm)
2/70b to scanner is 2.2W/1.2W

At 2' separation:
2/440+ to scanner is 9.2W/3.6W
2/70b to scanner is 5.1W/2.8W

Obviously 3' is preferred, but at 2', is 9.2W too dangerously close for an APRS beacon at regular intervals? By the numbers, either should work, if I'm looking at this correctly.

 

[prcguy]

I'm going to have to pull out a tape measure and verify the exact locations I think things can go.


I don't have all the materials for this change (and the SDS200 is tucked away nicely, making pulling it out to test that SMA connector pretty difficult) so I can't test it. I'm trying to do the front end planning and ruling out what won't work beforespend more money.




Using this Calculator Friis Transmission Equation Calculator
And the antenna information
Scanner NMO150/450/758: 2m-0.0dbi, 70cm-5dbi
Compactenna 2/440+: 5dbi
NMO 2/70b 2m-2.4dbi, 70cm-4dbi

At 3' separation:
2/440+ to scanner is 4W/1.6W (2 meters/70 cm)
2/70b to scanner is 2.2W/1.2W

At 2' separation:
2/440+ to scanner is 9.2W/3.6W
2/70b to scanner is 5.1W/2.8W

Obviously 3' is preferred, but at 2', is 9.2W too dangerously close for an APRS beacon at regular intervals? By the numbers, either should work, if I'm looking at this correctly.

 

[prcguy]

I think your antenna gain numbers are too high, I doubt if the NMO triband or COMPACtenna antennas exceed 0dBd or 2.14dBi gain. The NMO 2/70 has a tiny bit of gain.

Using this Calculator Friis Transmission Equation Calculator
And the antenna information
Scanner NMO150/450/758: 2m-0.0dbi, 70cm-5dbi
Compactenna 2/440+: 5dbi
NMO 2/70b 2m-2.4dbi, 70cm-4dbi

At 3' separation:
2/440+ to scanner is 4W/1.6W (2 meters/70 cm)
2/70b to scanner is 2.2W/1.2W

At 2' separation:
2/440+ to scanner is 9.2W/3.6W
2/70b to scanner is 5.1W/2.8W

Obviously 3' is preferred, but at 2', is 9.2W too dangerously close for an APRS beacon at regular intervals? By the numbers, either should work, if I'm looking at this correctly.

 

[vagrant]

What brand and model are you using that is rated for the 60 watts? The DXE 10W model does not look bad, but hey 60W.

I had one from DX Engineering rated up to 10 watts for HF use and I ended up blowing that one up due to too much coupling between antennas and too much power. I also picked up a couple rated 60 watts or so and now use that.

 

[prcguy]

I'm away from my main station for a few more days and don't recall the brand but I remember posting info and a picture a year or two back in another limiter thread. The units are a couple inches long and about 3/4in wide and designed to mount on a heatsink for extended operation. There was a short term rating and a long term depending on if you have a heat sink or not.

What brand and model are you using that is rated for the 60 watts? The DXE 10W model does not look bad, but hey 60W.

 

[prcguy]

I found the old thread and here is some data on the big limiters I have. It will handle 60 to 100w continuous with heatsink and up to 1kW peak for short duration pulses. ACLM-4896 - Cobham Signal & Control Solutions | Limiter

 

[tweiss3]

I think your antenna gain numbers are too high, I doubt if the NMO triband or COMPACtenna antennas exceed 0dBd or 2.14dBi gain. The NMO 2/70 has a tiny bit of gain.

Those are the manufacturer claims, 0/5/4dbi for the 150/450/758. The COMPACtenna claims 3db (I assumed dbi instead of dbd) for the origional 7", and 5db for the new 2/440+ 9" model. Even if I was over estimated, better to be "over" than under.

 

[tweiss3]

I found the old thread and here is some data on the big limiters I have. It will handle 60 to 100w continuous with heatsink and up to 1kW peak for short duration pulses. ACLM-4896 - Cobham Signal & Control Solutions | Limiter

60W continuous is pretty good.

I guess the next question is, how does one find out what the limit to the front end of a radio is without causing costly damage? It's not a published specification, but I would think there is a rule of thumb beyond "get as much separation as possible and pray its enough".

 

[vagrant]

Fudge...that ACLM-4896 can handle quite a bit. My pay grade would need to be a national Military, Space, or Commercial for the cost, but I am not the intended market. Looks like I already asked this question in the previous thread and forgot. The retail price must have caused me some mental trauma.

 

[mmckenna]

60W continuous is pretty good.

I guess the next question is, how does one find out what the limit to the front end of a radio is without causing costly damage? It's not a published specification, but I would think there is a rule of thumb beyond "get as much separation as possible and pray its enough".

I've seen 250mw as the suggested upper end. Never seen that published anywhere.

 

[tweiss3]

I've seen 250mw as the suggested upper end. Never seen that published anywhere.

That's a starting point. With 50W radios, and unity gain antennas, that requires a minimum of 11' separation for 2 meters and 4' for 70cm. Interesting.

 

[prcguy]

They were cheap on Ebay and I originally bought one then posted some info about them then bought another incase there was a run on them. Just keep checking Ebay, limiters of all kinds pop up here and there.

Fudge...that ACLM-4896 can handle quite a bit. My pay grade would need to be a national Military, Space, or Commercial for the cost, but I am not the intended market. Looks like I already asked this question in the previous thread and forgot. The retail price must have caused me some mental trauma.

 

[mmckenna]

That's a starting point. With 50W radios, and unity gain antennas, that requires a minimum of 11' separation for 2 meters and 4' for 70cm. Interesting.

With commercial radios, there's usually enough filtering on the front ends to prevent that from being an issue. I've had VHF, UHF and 800MHz radios installed on trucks and no issues with a few feet of separation. Just look the rooftop of a CHP vehicle, and they do OK with 100 watt radios and only a foot or 2 separation between antennas.

Not so much when it comes to the wide band RX ham radios and scanners.

 

[tweiss3]

They were cheap on Ebay and I originally bought one then posted some info about them then bought another incase there was a run on them. Just keep checking Ebay, limiters of all kinds pop up here and there.

I couldn't find a price, but based on the above comments, I'm guessing its is as much as a radio.

With commercial radios, there's usually enough filtering on the front ends to prevent that from being an issue. I've had VHF, UHF and 800MHz radios installed on trucks and no issues with a few feet of separation. Just look the rooftop of a CHP vehicle, and they do OK with 100 watt radios and only a foot or 2 separation between antennas.

Not so much when it comes to the wide band RX ham radios and scanners.

This thread made me shoot a few emails to manufacturers about radio front ends. The responses I've received are very interesting, but not exactly helpful.