I have a covered deck off my house right beside the radio room. The crossbeams are 8' off the floor and the peak of the roof is 6' above the crossbeam. I got the idea to put an antenna on one of the crossbeams so that during storms, I can unplug from the "outdoor" 36' antenna and use the "indoor" deck antenna to reduce the likelihood of lightning damage. I was using a mag-mount mobile antenna on a 16" pizza plate and running the 16' RG58 coax through the corner of the door frame into the house and it worked great. Using a Comet 2x4SR NMO antenna, I got excellent SWRs and good tx/rx on GMRS, 2 meters, 70 centimeters, and (somewhat surprisingly) 1.25 meters.
This week, I decided to do a more permanent installation. I built a 36" x 36" platform which I covered with aluminum and mounted on one of the crossbeams. I installed an NMO connector and ran 25' of KMR400 coax to my window pass-through panel. When I hooked up the NanoVNA, to my dismay, SWR on GMRS was 3.0+, 1.25m was 2.75+, and MURS was 3.0+. SWRs on 2m and 70cm were ~1.2:1. It's probably worth noting that these are the two bands the antenna is actually designed to operate in. I screwed the antenna back onto the mag-mount, placed the mount in the center of the platform, and retested. All the SWRs were back well below 2.0:1. The ground plane remained the same, so the only things which might account for the difference were the connector and the coaxial cable.
It's known that a lossy cable will often result in a lower measured SWR because any RF energy lost in the cable won't be reflected back to the radio. However, a lossy cable will also result in a reduction in RF output from the antenna, so we typically avoid using it, if possible, but that doesn't mean you can't or even shouldn't use it, depending upon the circumstances. My concern about high SWRs is the possibility of the reflected power damaging the radio. We could argue all day about how likely that is to happen, but I don't want to risk it.
I won't bore you with the details of the various cable combinations I tested, but I found that with 25' of RG8X from the antenna to the window pass-through and 6' of RG8X from the pass-through connector to the radio, all the SWRs were below 2.0:1. The SWR on 467 MHz improved even more using RG58, but "good enough" is a valid concept and I stuck with the RG8X.
So what about loss of signal strength? Well, a little context is important here. The 2m and 70cm repeaters I'd most want to reach during severe weather -- the ones which host the local ham severe weather nets -- can be reached from my yard with a 5 watt HT. The two GMRS repeaters nearest me can be reliably reached from my yard with 10 watts. My base radio is capable of up to 50 watts, but I usually run it on ~12 watts (measured). My backup unit (should I decide to use it) outputs a measured 20-27 watts depending upon the frequency. The reported gain of the Comet 2x4SR antenna is 6.2 dBi (4.05 dBd) on UHF and 3.8 dBi (1.65 dBd) on VHF. Using an SWR of 2.0:1 (worst-case scenario, since they're all actually at least a little better than that), 20 watts of RF output, and 31' of RG8X, plugging the numbers into a coax loss calculator gives me an effective radiated power of 42.1 watts on 467 MHz and 24.3 watts on 146 MHz; more than enough power for what I want to accomplish.
This is kind of long, but the point is that intentionally using a lossy cable made it feasible to use a single antenna to cover all the bands I'm interested in. The RG8X is lossy, but running the numbers shows that it won't drop the ERP below what I actually need. Caveat: Yes, I know getting a good SWR doesn't mean the antenna will actually function well on a given frequency. I had this antenna on my car for several months and I know it performs well on GMRS, 70cm, 1.25m, and 2m because I verified it in real-world use. Using a lossy cable to improve SWR won't make the antenna tx/rx effectively in a frequency band where it didn't work before, but it will make it possible to use it without concern for my radio.
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I have a covered deck off my house right beside the radio room. The crossbeams are 8' off the floor and the peak of the roof is 6' above the crossbeam. I got the idea to put an antenna on one of the crossbeams so that during storms, I can unplug from the "outdoor" 36' antenna and use the "indoor" deck antenna to reduce the likelihood of lightning damage. I was using a mag-mount mobile antenna on a 16" pizza plate and running the 16' RG58 coax through the corner of the door frame into the house and it worked great. Using a Comet 2x4SR NMO antenna, I got excellent SWRs and good tx/rx on GMRS, 2 meters, 70 centimeters, and (somewhat surprisingly) 1.25 meters.
This week, I decided to do a more permanent installation. I built a 36" x 36" platform which I covered with aluminum and mounted on one of the crossbeams. I installed an NMO connector and ran 25' of KMR400 coax to my window pass-through panel. When I hooked up the NanoVNA, to my dismay, SWR on GMRS was 3.0+, 1.25m was 2.75+, and MURS was 3.0+. SWRs on 2m and 70cm were ~1.2:1. It's probably worth noting that these are the two bands the antenna is actually designed to operate in. I screwed the antenna back onto the mag-mount, placed the mount in the center of the platform, and retested. All the SWRs were back well below 2.0:1. The ground plane remained the same, so the only things which might account for the difference were the connector and the coaxial cable.
It's known that a lossy cable will often result in a lower measured SWR because any RF energy lost in the cable won't be reflected back to the radio. However, a lossy cable will also result in a reduction in RF output from the antenna, so we typically avoid using it, if possible, but that doesn't mean you can't or even shouldn't use it, depending upon the circumstances. My concern about high SWRs is the possibility of the reflected power damaging the radio. We could argue all day about how likely that is to happen, but I don't want to risk it.
I won't bore you with the details of the various cable combinations I tested, but I found that with 25' of RG8X from the antenna to the window pass-through and 6' of RG8X from the pass-through connector to the radio, all the SWRs were below 2.0:1. The SWR on 467 MHz improved even more using RG58, but "good enough" is a valid concept and I stuck with the RG8X.
So what about loss of signal strength? Well, a little context is important here. The 2m and 70cm repeaters I'd most want to reach during severe weather -- the ones which host the local ham severe weather nets -- can be reached from my yard with a 5 watt HT. The two GMRS repeaters nearest me can be reliably reached from my yard with 10 watts. My base radio is capable of up to 50 watts, but I usually run it on ~12 watts (measured). My backup unit (should I decide to use it) outputs a measured 20-27 watts depending upon the frequency. The reported gain of the Comet 2x4SR antenna is 6.2 dBi (4.05 dBd) on UHF and 3.8 dBi (1.65 dBd) on VHF. Using an SWR of 2.0:1 (worst-case scenario, since they're all actually at least a little better than that), 20 watts of RF output, and 31' of RG8X, plugging the numbers into a coax loss calculator gives me an effective radiated power of 42.1 watts on 467 MHz and 24.3 watts on 146 MHz; more than enough power for what I want to accomplish.
This is kind of long, but the point is that intentionally using a lossy cable made it feasible to use a single antenna to cover all the bands I'm interested in. The RG8X is lossy, but running the numbers shows that it won't drop the ERP below what I actually need. Caveat: Yes, I know getting a good SWR doesn't mean the antenna will actually function well on a given frequency. I had this antenna on my car for several months and I know it performs well on GMRS, 70cm, 1.25m, and 2m because I verified it in real-world use. Using a lossy cable to improve SWR won't make the antenna tx/rx effectively in a frequency band where it didn't work before, but it will make it possible to use it without concern for my radio.
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