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Why "More Power" Isn't The Answer


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#21 RCM

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Posted 13 December 2019 - 12:31 PM

Yep, I can certainly vouch for that. Since putting a super high gain antenna high up, the analyzer was reading a massive noise threshold.... needed 2 cans (cavities) to bring it down to an "acceptable" level.

 

I think the radio range these days, aside from more TX power, is also limited by receiver being overloaded by RF noise; b/c after finding out the noise threshold at my location I was blown away how high it was... so if you don't have a radio that has a tight front end you won't receive signals from very far...

 

Don't sell yourself short, Marc, you are been helpful to others, and nobody knows everything.... point is that sometimes little tidbits like that are the eureka moment that makes it, even for knowledgeable ppl.

 

G.

Good points, but not everyone lives/works in an area with such a high noise floor.



#22 WRAF213

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Posted 13 December 2019 - 02:55 PM

Yep, I can certainly vouch for that. Since putting a super high gain antenna high up, the analyzer was reading a massive noise threshold.... needed 2 cans (cavities) to bring it down to an "acceptable" level.

 

I think the radio range these days, aside from more TX power, is also limited by receiver being overloaded by RF noise; b/c after finding out the noise threshold at my location I was blown away how high it was... so if you don't have a radio that has a tight front end you won't receive signals from very far...

Cavity filters don't actually make noise go away, they just reduce out-of-band interference. You should have no difference on your noise floor power (aside from filter loss) on GMRS after installing properly tuned cavity filters. Are you measuring out-of-band transmitters as being part of your noise floor? Those would be interference sources, not noise, in the grand scheme of things; noise sources would be unintentional radiators or some malfunctioning transmitters. A noise power exceeding -60dBm over 100 kHz bandwidth would be nonsensical, but it's pretty sane to have a -60dBm signal somewhere within a particular 100 kHz bandwidth on a fixed setup.

 

nsNmV6Ml.png



#23 gman1971

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Posted 13 December 2019 - 05:11 PM

I used the wrong words, sorry. I usually call noise, wrongly, to anything that doesn't let me hear the signals I need. And Having a giant 1400 foot tall tower 2 miles away from my house, with 8 different TV stations, NOAA radios, police dispatch, and who knows what other fire breathing RFIs coming out of it... the cavities did indeed help with the receiver being swamped with garbage from all over the spectrum. 

 

G.

 

 

Cavity filters don't actually make noise go away, they just reduce out-of-band interference. You should have no difference on your noise floor power (aside from filter loss) on GMRS after installing properly tuned cavity filters. Are you measuring out-of-band transmitters as being part of your noise floor? Those would be interference sources, not noise, in the grand scheme of things; noise sources would be unintentional radiators or some malfunctioning transmitters. A noise power exceeding -60dBm over 100 kHz bandwidth would be nonsensical, but it's pretty sane to have a -60dBm signal somewhere within a particular 100 kHz bandwidth on a fixed setup.

 

nsNmV6Ml.png



#24 gman1971

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Posted 13 December 2019 - 05:13 PM

I understand that, and that is good, b/c living in this kind of area sucks for radio comms.

 

G.

 

Good points, but not everyone lives/works in an area with such a high noise floor.


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#25 WRAF213

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Posted 13 December 2019 - 09:01 PM

Yeah, being within that distance of a broadcaster is the real problem at hand. Based on your symptoms, I'm assuming there's UHF TV broadcast on that tower in the 500 MHz band (which is almost all that's left of UHF TV). TV typically runs tens to hundreds of kilowatts, and any internal or external mixing products of TV broadcast may resemble noise due to the DTV modulation. FM is more easily controlled and the harmonics aren't in-band (fourth harmonic ends at 432.2 MHz), but that still requires strong filtering. To do an actual check of your RF environment and noise floor, you'd need a good spectrum analyzer that stays linear up to at least 10dBm; most quality equipment is rated up to +30dBm.

 

If you've got noise that appears to be in-band but cavity filters improve the situation, you've probably got intermod going on. Band-reject filters near the antenna can help with that. If filtering near the receiver doesn't improve in-band performance to a satisfactory level, PIM may be at play. Right now, intermod and overload effects could mask a generally crap noise floor from unintentional radiators; don't expect to dive in with hundreds of dollars of equipment and hit thermal noise floor (though you'll have to spend a little bit more to avoid PIM and ensure linearity of any active components). A noise power of -125dBm/16 kHz is pretty typical for a quiet area, compared to thermal noise floor at -131dBm/16 kHz. HDMI/DVI are particularly egregious noisemakers on UHF LMR bands, especially with AliExpress-quality cables. LMR-400 would make a mess of your RF environment.

 

Overload effects don't cause CCRs to not be heard, it simply knocks out their ability to hear. Any RDA1846 design will show desense on almost any rooftop antenna anyways; they're meant to be used with unity-gain antennas attached to the unit and work rather well in that scenario. Desense on the stock antenna or equivalent is only encountered in rather exceptional RF environments.


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#26 RCM

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Posted 13 December 2019 - 11:55 PM

I understand that, and that is good, b/c living in this kind of area sucks for radio comms.

 

G.

Man, I bet it does.



#27 gman1971

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Posted 14 December 2019 - 12:48 AM

Here it is... Madison Candelabra Tower... aka, RFI fire breathing from hell...

 

http://www.sbe24.org...andel-south.asp

 

Yeah, CCRs don't hear squat near the tower... the better antenna you use, the worse the problem gets... :D

 

Nearly all LMR400 is gone from my base setup (just one patch cable left). I run heliax 1/2 only. Nearly all my connectors are trimetal too...  still a couple of UHF nickel plated, antenna and radio connector... I don't understand why Motorola went with the mini-UHF connectors... such a hassle to deal with those instead of N connectors.

 

G.

 

G.



#28 WRAF213

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Posted 16 December 2019 - 03:19 AM

Doing some research on that tower shows three UHF TV transmitters up on the top (ignoring the lower-power VHF transmitters).

WMSN-TV: 440kW at 497 MHz

WHA-TV: 200kW at 509 MHz

WKOW-TV: 800kW at 545 MHz

 

No wonder you're having problems. Path loss is roughly 100dB at that distance, since there doesn't appear to be any terrain obstruction anywhere within 2 miles of that tower. With transmit power in the ballpark of 80-85dBm ERP towards your elevation, that's a LOT of leftover signal that's as close as 30 MHz to GMRS (about the distance from GMRS to 70cm).



#29 gman1971

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Posted 16 December 2019 - 07:04 AM

Yup, I live less than 2 miles away from that tower, and the VHF side is nothing to sneeze at either.... welcome to the jungle...



#30 n4gix

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Posted 29 December 2019 - 03:47 PM

Good grief! That is one awesome - but rather overpopulated - tower. I'm happy to have nothing that obscene anywhere near me! :lol:


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#31 RCM

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Posted 29 December 2019 - 04:35 PM

Good grief! That is one awesome - but rather overpopulated - tower. I'm happy to have nothing that obscene anywhere near me! :lol:

Me too. I think that would be enough to make me move to a better location.

You know how some people talk about "location, location, location?" This is what they were talking about! :blink:



#32 gman1971

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Posted 30 December 2019 - 02:31 AM

Location... lol... the location aint bad, that is why the put the darn tower here... hahaha...

 

I've considered moving out of here at some point, but since we just moved in 3 years ago, looks like I might be stuck for a couple more years at the very least... and moving will require more like moving half state away... b/c the tower is still visible line of sight from 30 miles away. I can see its red beacons 30 miles when coming from Milwaukee... 

 

Oh well.

 

G.



#33 Jones

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Posted 30 December 2019 - 09:40 AM

Hey Gman1971,

 

If you need to clean up that RF overload issue at your site a bit, I have had great luck at my Ham club's repeater sites with Olds Communication Inc bandpass filters.  We used to have issues at our 444.475 site with overload from the local ambulance paging service (on same tower) running 500 Watts on MED-9 (462.950). An Olds Ham UHF bandpass filter solved the problem.  Likewise at one of our 2-Meter sites, 147.210, we had trouble with a NWS/NOAA station running 1KW on 162.550 that an Olds VHF filter helped out tremendously.

 

I don't have any business interest in this company, and I am not being paid to advertise.  I just use these products, and know they work as promised.

 

https://www.ocicom.com/index.php

 

Click on the "Commercial" tab, and you will get the number to talk to Ralph Olds directly, and you can tell him what bandpass and bandwidth you need.


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#34 gman1971

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Posted 30 December 2019 - 06:20 PM

Wow, those look super nice. Thank you, Jones!

 

Looks like those could be the answer for a mobile environment too!!

 

G.


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#35 Jones

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Posted 30 December 2019 - 11:46 PM

Wow, those look super nice. Thank you, Jones!

 

Looks like those could be the answer for a mobile environment too!!

 

G.

I don't see any reason why they wouldn't work in a mobile installation, other than size restraints.  The UHF one is about as big as a cigar box, but if you have room to stow it, it should certainly work.  Of course, you wouldn't want to use one of these on a dual-band radio, unless you have split UHF/VHF transmit ports, and split antennas.

 

I think they would really shine in a mobile installation (or base station) where you had a 70cm Ham rig, and a separate GMRS rig, with close-mounted antennas.  If you have one of these tuned filters on each radio, you should be able to transmit on GMRS without blanking out your 70cm rig, and vice-versa.


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#36 gman1971

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Posted 31 December 2019 - 01:08 AM

I don't see any reason why they wouldn't work in a mobile installation, other than size restraints.  The UHF one is about as big as a cigar box, but if you have room to stow it, it should certainly work.  Of course, you wouldn't want to use one of these on a dual-band radio, unless you have split UHF/VHF transmit ports, and split antennas.

 

I think they would really shine in a mobile installation (or base station) where you had a 70cm Ham rig, and a separate GMRS rig, with close-mounted antennas.  If you have one of these tuned filters on each radio, you should be able to transmit on GMRS without blanking out your 70cm rig, and vice-versa.

 

Wow, that is small indeed. Insertion loss is rather low considering it provides 70+ dB attenuation for out of band signals.. 

 

Yeah, I will be calling tomorrow to see how much for a GMRS filter. Wondering if I should get one just for the 467 or the 462? or one for each? Since there is a whole 5 mhz of noise making RF spectrum between the GMRS frequencies.

 

G.



#37 Jones

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Posted 31 December 2019 - 09:12 AM

Wow, that is small indeed. Insertion loss is rather low considering it provides 70+ dB attenuation for out of band signals.. 

 

Yeah, I will be calling tomorrow to see how much for a GMRS filter. Wondering if I should get one just for the 467 or the 462? or one for each? Since there is a whole 5 mhz of noise making RF spectrum between the GMRS frequencies.

 

G.

I was thinking that was too big to fit anywhere in my car, but then I drive a Nissan Juke.  I don't have room for a cigar box.

 

Also, I wasn't thinking about the noise in between the GMRS split, but you're right.  For instance, that MED-9 repeater that I had to fight with on the 444.475 tower.  That is just above GMRS, at 462.950/467.950.  I guess the only way to filter that might be with additional P/R cavity filters, and those add more loss per each can.  These Olds filters could certainly be designed to block out all of the other business and public safety stuff in the 450-460 MHz and 470-512 MHz ranges, as well as all that UHF TV stuff near you.  Once you lower that noise, the other stuff in-band around 463-465 might not bother you as much.

 

Also, the fairly wide bandwidth is a feature of these Olds filters. The ham version allows you to work the whole 430-449 range without having to worry about re-tuning anything.


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#38 gman1971

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Posted 31 December 2019 - 08:51 PM

LOL, cigar box...

 

Cavities seem to work well for repeater operation, but not for general purpose b/c as you said, they require tuning... 

 

Might be worth just on the RX side for the stuff, on the 467 side, since the stuff that listens would be portables with a small portable antenna, which seems to be more forgiving than a high gain base antenna...

 

G.

 

I was thinking that was too big to fit anywhere in my car, but then I drive a Nissan Juke.  I don't have room for a cigar box.

 

Also, I wasn't thinking about the noise in between the GMRS split, but you're right.  For instance, that MED-9 repeater that I had to fight with on the 444.475 tower.  That is just above GMRS, at 462.950/467.950.  I guess the only way to filter that might be with additional P/R cavity filters, and those add more loss per each can.  These Olds filters could certainly be designed to block out all of the other business and public safety stuff in the 450-460 MHz and 470-512 MHz ranges, as well as all that UHF TV stuff near you.  Once you lower that noise, the other stuff in-band around 463-465 might not bother you as much.

 

Also, the fairly wide bandwidth is a feature of these Olds filters. The ham version allows you to work the whole 430-449 range without having to worry about re-tuning anything.



#39 n4gix

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Posted 01 January 2020 - 01:43 PM

I use similar filters for HF on Field Day and Parks on the Air events. If I'm going to work 20m I put a 20m passband filter in-line and the other guys on different bands don't swamp my 20m reception. They average -40db rejection outside of the band they are designed for.
 
They aren't inexpensive ($84/each), so the club has a single set of 1/each for every ham band.

https://www.dunestar...ndpass-filters/



#40 gman1971

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Posted 02 January 2020 - 02:29 AM

Filters seem to be the answer to the crowded RF space... I also found that grounding the antenna and the mast helped quite a bit.

 

G.






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