Improving SWR by using lossy coax

[K9KLC]

This was a workaround for a specific situation to try to get acceptable performance from a single antenna on the 2m, 1.25m, 70cm, and GMRS bands in a low-power, infrequent-use case. My "real" 2m, 1.25m, and 70cm base antenna is on a 36' mast with LMR400 coax, and my "real" GMRS antenna is on a 50' mast also with LMR400, but I unplug from them when storms are in the area.

This all makes a little more sense to me now what you're trying to get working there. Good luck.

 

[OkieBoyKJ5JFG]

Got it. The PL-259 on the coax is another possible source of an "impedance bump" which might affect your SWR.

It sure could be, but I think the biggest issue is that I'm trying to use an antenna on a band (GMRS) that it wasn't actually designed for. I'm using a workaround to fool the radio into not blowing up, conceptually similar to an antenna tuner. If I didn't want to get GMRS and 2 meters in the same antenna, it wouldn't be an issue. It's not practical to put two antennas in my "storm shelter" location, but I don't want to give up the ability to talk to my GMRS friends when the weather makes me unplug my "real" antennas. A local "prepper" group runs a net of sorts during severe weather and I like to stay in touch.

I know the antenna transmits well on GMRS because I used it for months in the same location with a smaller ground plane fed by 16' of RG58 on a mag mount. SWR on all bands was great, but I had to move the radios out of the shack to connect to it, so I'm routing the antenna to my window pass-through. I know it's resonant enough for my purposes, I just need to get the SWR down to something I'm comfortable with, and I don't really want to buy a new antenna or anything else expensive for such a niche use case. Besides, it's kinda fun to experiment.

 

[mmckenna]

There's other options for mobile antennas that will cover those bands will more than acceptable SWR. But if you've got a work around, go with it.

 

[OkieBoyKJ5JFG]

There's other options for mobile antennas that will cover those bands will more than acceptable SWR. But if you've got a work around, go with it.

Yeah, but I'd have to research it and buy one. Then, with my luck, I wouldn't have the right coax for that antenna to work in my application.

 

[mmckenna]

Yeah, but I'd have to research it and buy one. Then, with my luck, I wouldn't have the right coax for that antenna to work in my application.

If you have an NMO mount, you have what you need.

1/4 wave VHF whip will easily cover 2 meters, MURS and everything else on RX. About $15.00
That'll also be 3/4 wave on UHF, and will work GMRS and 70cm just fine with a bit of a less than ideal radiation pattern. But if you can work these repeaters with a hand held, it doesn't sound like you need a lot of antenna.

Or, step up to one of the multiband NMO antennas. VHF & UHF, or VHF, UHF & 7/8/900MHz.

I've used all of those and swept them with antenna analyzers. SWR is acceptable across the ham bands, GMRS and the LMR bands.

 

[OkieBoyKJ5JFG]

So...this is my final answer...I think...

I used a Diamond SG7900 "Super Gainer" with 3.85 dBd gain on VHF and 6.45 dBd gain on UHF (calculated from the manufacturer's reported dBi gain). I used 25' of RG8X from to the antenna to the pass-through and 20' of RG58 from the pass-through to the radio and got SWRs below 1.5:1 on GMRS, 2m, and 70cm. I don't care about 1.25m because, like the Dread Pirate Roberts, that repeater is Mostly Dead and I won't be using it in severe weather anyway.

If my calculations are correct (figuring loss from the RG58 and calculating watts fed to the RG8X, then using that number to calculate loss through the RG8X), 20 watts of input from the radio should translate into about 30 watts on 146 MHz, 35 watts on 443 MHz, and 34 watts on 467 MHz ERP from the antenna. That's more than adequate for my needs, bearing in mind that I can hit all those repeaters with a 10 watt HT from my yard, but not reliably from inside the house. Being under the roof of the deck will attenuate it a little, but likely not as much as being inside the house, so 30 watts will probably blast right through, even during weather with a lot of electrical activity.

 

[OkieBoyKJ5JFG]

If you have an NMO mount, you have what you need.

1/4 wave VHF whip will easily cover 2 meters, MURS and everything else on RX. About $15.00
That'll also be 3/4 wave on UHF, and will work GMRS and 70cm just fine with a bit of a less than ideal radiation pattern. But if you can work these repeaters with a hand held, it doesn't sound like you need a lot of antenna.

Or, step up to one of the multiband NMO antennas. VHF & UHF, or VHF, UHF & 7/8/900MHz.

I've used all of those and swept them with antenna analyzers. SWR is acceptable across the ham bands, GMRS and the LMR bands.

Good to know. I'll keep that in mind. I think for now, I have a working solution with something I already had, but it's not unlikely I'll decide to tinker with it again sometime in the future. I'm just that way.

 

[OkieBoyKJ5JFG]

But if you can work these repeaters with a hand held, it doesn't sound like you need a lot of antenna.

No, I don't need much. I just don't want people to think I don't have enough sense to get out of the rain.

 

[K9KLC]

So...this is my final answer...I think...

I used a Diamond SG7900 "Super Gainer" with 3.85 dBd gain on VHF and 6.45 dBd gain on UHF (calculated from the manufacturer's reported dBi gain). I used 25' of RG8X from to the antenna to the pass-through and 20' of RG58 from the pass-through to the radio and got SWRs below 1.5:1 on GMRS, 2m, and 70cm. I don't care about 1.25m because, like the Dread Pirate Roberts, that repeater is Mostly Dead and I won't be using it in severe weather anyway.

If my calculations are correct (figuring loss from the RG58 and calculating watts fed to the RG8X, then using that number to calculate loss through the RG8X), 20 watts of input from the radio should translate into about 30 watts on 146 MHz, 35 watts on 443 MHz, and 34 watts on 467 MHz ERP from the antenna. That's more than adequate for my needs, bearing in mind that I can hit all those repeaters with a 10 watt HT from my yard, but not reliably from inside the house. Being under the roof of the deck will attenuate it a little, but likely not as much as being inside the house, so 30 watts will probably blast right through, even during weather with a lot of electrical activity.

I've got a 7900 on my old work truck, well over 20 years old and well beat up. That thing just keeps going and going. I've never used it on MURS but I have on GMRS and while it was almost 2.7:1 there my old Kenwood TK805D didn't blink an eye for the month or so I did GMRS with it. I'm back to doing ham frequencies on it now but it did work there. I think it was only about 10 or so feet of RG-316 Coax on the thing, Needed the skinny stuff to make it thru the hood mount.

 

[mmckenna]

Good to know. I'll keep that in mind. I think for now, I have a working solution with something I already had, but it's not unlikely I'll decide to tinker with it again sometime in the future. I'm just that way.

Part of the fun of the hobby.
I've been doing the same thing for several decades.

After a while, I gave up on the gimmicky ham antennas and started using the commercial stuff. Sometimes it's actually cheaper. I've found that 1/4 wave antennas give me what I need and the bandwidth required to use the Part 90 radios I have for both work and amateur radio.

 

[OkieBoyKJ5JFG]

Part of the fun of the hobby.
I've been doing the same thing for several decades.

After a while, I gave up on the gimmicky ham antennas and started using the commercial stuff. Sometimes it's actually cheaper. I've found that 1/4 wave antennas give me what I need and the bandwidth required to use the Part 90 radios I have for both work and amateur radio.

I'm new to this and still very much a hobbyist. Right now, I'm trying to use up stuff I bought that didn't work out to be quite what I wanted. The SG7900 worked great on my truck, but I changed to a 2m/1.25m/70cm radio and needed a different antenna. If I can make it work for my "severe weather antenna" that's a bonus. I don't really know why it had great SWRs on GMRS with the mag mount, pizza plate, and 16' of RG58, but perhaps it's because the NMO connector on the mag mount is directly connected to the coax and the one on my platform isn't. However, the 16' of coax wouldn't reach my window pass-through, so I had to do something else. I did check for continuity on the NMO connector, and it was fine. I'm sure all connectors affect SWR to some degree, but this seemed a bit much.

 

[bunangst]

Coming from the public safety Communications Industry, radios do not get unplugged for lightning. I suggest using your time and effort for proper grounding and surge protection devices. Do some reading into NEC and Motorola R56.

 

[OkieBoyKJ5JFG]

Coming from the public safety Communications Industry, radios do not get unplugged for lightning. I suggest using your time and effort for proper grounding and surge protection devices. Do some reading into NEC and Motorola R56.

I have installed grounding and surge protection in compliance with NEC and local codes. I unplug in addition to that, not instead of it. Maybe that's overkill, but it costs nothing and it makes me feel safer.

 

[K9KLC]

Coming from the public safety Communications Industry, radios do not get unplugged for lightning. I suggest using your time and effort for proper grounding and surge protection devices. Do some reading into NEC and Motorola R56.

Coming from owning repeaters I never unhooked them in storms. Would have had to drive probably a couple hundred miles to get them all unhooked and then rehooked back up. Yep took a strike twice but it took out the controller and the culprit was the auto patch board both times.

 

[OkieBoyKJ5JFG]

Coming from owning repeaters I never unhooked them in storms. Would have had to drive probably a couple hundred miles to get them all unhooked and then rehooked back up. Yep took a strike twice but it took out the controller and the culprit was the auto patch board both times.

Yeah, but these antennas aren’t 100 miles from my house; they’re literally in my house. I’m not too worried about the equipment, but I’d sure hate to be homeless. Then I’m back to standing outside inside a storm.

 

[prcguy]

Got it. The PL-259 on the coax is another possible source of an "impedance bump" which might affect your SWR.

Yes but no, not really. Not below about 500 MHz unless it’s put on wrong.

 

[OkieBoyKJ5JFG]

Yes but no, not really. Not below about 500 MHz unless it’s put on wrong.

Can you elaborate on that, please? I'm a rookie.

 

[AK9R]

Depending on the source of your information, the PL-259 connector has a bit of loss and impedance mis-match at higher frequencies. Some will say the PL-259 is a horrible connector and should not be allowed in any radio installation. Some say there's nothing wrong with the connector as long as you understand its limitations.

That said, for most applications, if the PL-259 is properly installed, it usually isn't a problem on 144 and 440 MHz amateur radio installations. I emphasize "properly installed". I hedged my comment yesterday by saying "possible source". @prcguy got right to the point by saying "unless it’s put on wrong".

Installing a PL-259 on a piece of RG-8, or similar, coax is an amateur radio right of passage. Especially, if it's the solder type of connector. Getting solder to flow between the body of the connector and the shield of the coax without melting the dielectric is a challenge. It's my understanding that the the telecommunications industry has gone to the crimp type connectors. I ass-u-me they've done so because the crimp type is easier and faster to install in the field than solder type with less chance of making a poor connection.

If your coax has PL-259 connectors that were factory crimped or crimped by an experienced person in the field using the correct tools, then your PL-259s are probably OK for your application. If they were soldered in the field, then they might, emphasize "might", cause a problem.

Two interesting websites:

The Unfair Maligning of the PL-259

www.n1gy.com

The PL259 myth

You often read or hear that every PL259 connector on your coax adds 0,5dB loss. This might be the truth on UHF and higher. But on HF/VHF tha...

pe4bas.blogspot.com

 

[OkieBoyKJ5JFG]

Depending on the source of your information, the PL-259 connector has a bit of loss and impedance mis-match at higher frequencies. Some will say the PL-259 is a horrible connector and should not be allowed in any radio installation. Some say there's nothing wrong with the connector as long as you understand its limitations.

That said, for most applications, if the PL-259 is properly installed, it usually isn't a problem on 144 and 440 MHz amateur radio installations. I emphasize "properly installed". I hedged my comment yesterday by saying "possible source". @prcguy got right to the point by saying "unless it’s put on wrong".

Installing a PL-259 on a piece of RG-8, or similar, coax is an amateur radio right of passage. Especially, if it's the solder type of connector. Getting solder to flow between the body of the connector and the shield of the coax without melting the dielectric is a challenge. It's my understanding that the the telecommunications industry has gone to the crimp type connectors. I ass-u-me they've done so because the crimp type is easier and faster to install in the field than solder type with less chance of making a poor connection.

If your coax has PL-259 connectors that were factory crimped or crimped by an experienced person in the field using the correct tools, then your PL-259s are probably OK for your application. If they were soldered in the field, then they might, emphasize "might", cause a problem.

Two interesting websites:

The Unfair Maligning of the PL-259

www.n1gy.com

The PL259 myth

You often read or hear that every PL259 connector on your coax adds 0,5dB loss. This might be the truth on UHF and higher. But on HF/VHF tha...

pe4bas.blogspot.com

Okay, good to know. I typically go with PL259/SO239 because that's what most equipment comes with. I do test them for continuity if I install them myself, but I'm not sure if that alone is enough to tell me if there will be an issue with mismatch.

 

[OkieBoyKJ5JFG]

As luck would have it, we have rain in the area today, so I checked in to the local 2 meter net with my "storm antenna" and my backup radio on battery. Reportedly, I got in just as well as I usually do with my AT-5888UVIII and a CX-333 antenna on a 36' mast, so that's encouraging. Of course, I can reach that repeater with a 10 watt HT from my yard. I'm sure the more distant ones will be out of reach, but it's the local ones I really want to be able to reach during severe weather, so I'm counting this as a success. This system isn't meant to replace the "real" antenna and radio; it's just so I have options. And, TBH, because I had unused antennas, coax, and plywood lying around and wanted to see what I could do with it.