Why do I get 2 different swr readings when I switch radios?

[prcguy]

The second harmonic would have to be really bad for this to happen. If the second harmonic were only 10dB down from the primary frequency and you had 100% reflection back from the antenna at the second harmonic, and you have very low loss coax you would read about a 1.9:1 SWR with an otherwise perfect match at the primary frequency. A higher SWR reading from a second harmonic would mean the radio is putting out a lot of power out of band.

PRCGUY is probably on it with regards to harmonics.
It could be a 2ND harmonic issue with the ic706 when used on cb which is out of its desgned band giving a improper VSWR indication or it could also be a spurious non harmonic issue with IC706 WHEN used in the CB band, what does the 706's spectrum look like at 27 MHZ as compared to 28 MHZ?

When the ic706 is modified for cb use, even at low power output, what does the 2nd harmonic and other non harmonic spurious look like compared to the HR2510?

2Fo should be around -50dBc, when you operate a hacked ham transceiver out of its designed band and you really should look at the harmonics and spurious emissions out of the transmitter with a spectrum analyzer.

 

[wowologist]

ya what mm said above is exactly what is happening. Amateur radios may be "modded" for out of band operation but ther is literally zero filtering and for many many years back in the day the FCC demanded that manufacturers include circuitry to cause extreme damage to the finals of the radio being operated illegally in the 11m band. This was done so Zed couldn't grab a 100w ham rig and blah blah blah from his truck ..easily. (not to mention the numerous zillions of amps that have been created over time for just such illegal, immoral activities....) And if that is an early model 706 ~20+ is years old...if you blow those finals its a shoe box as they are EOL and their is no drop in replacement without major board surgery and at that stage its not worth it for a 150$ swap meet rig nowadays. I dont know why someone would want to use a ham rig on 11m these days, you can buy one for like 29.95$ - but alas to each his own...

 

[prcguy]

All amateur radios I know of over the last 40yrs or so have band pass filters that switch in the right places for all frequencies from 1.6 to 30Mhz when the radio is modded for wide band operation. I have had four Icom 706 series radios and still run a 706MKIIG in my truck. It works beautifully out of band with no more harmonics than operating within the ham bands. That is by design as the radio mfrs know many of their radios will be used out of band.

The Icom 706MKIIG is not a $150 swap meet rig, they still fetch over $500 in many cases because its a very very good radio for mobile use and much better than its replacement the IC-7000 and many newer radios sold today. If and when it breaks (doubtful) I have a new IC-7100 to replace it and the 706 has about the same performance.

ya what mm said above is exactly what is happening. Amateur radios may be "modded" for out of band operation but ther is literally zero filtering and for many many years back in the day the FCC demanded that manufacturers include circuitry to cause extreme damage to the finals of the radio being operated illegally in the 11m band. This was done so Zed couldn't grab a 100w ham rig and blah blah blah from his truck ..easily. (not to mention the numerous zillions of amps that have been created over time for just such illegal, immoral activities....) And if that is an early model 706 ~20+ is years old...if you blow those finals its a shoe box as they are EOL and their is no drop in replacement without major board surgery and at that stage its not worth it for a 150$ swap meet rig nowadays. I dont know why someone would want to use a ham rig on 11m these days, you can buy one for like 29.95$ - but alas to each his own...

 

[bill4long]

Waiting for the dummy load results. 706s are fully capable of 11 meter operation without issue. Now, the OP has not actually said what freq he was testing on. Maybe he doesn't want to and I don't blame him. But dollars to donuts that isn't the problem.

 

[fantasma25]

Do you have a resistive-only dummy load handy? If so, put it on the antenna side of the SWR meter and check both radios just for funzies.

Unfortunately I do not have one yet, in process of getting one.

 

[fantasma25]

It could be the antenna is tuned with the HR2510 in line with specific jumper cables, etc. Certain antennas like the A99 and others are not decoupled very well from the feedline and the feedline and its length can be part of the tuned antenna system. Maybe when the Icom 706 is in line it has a different jumper cable length or something else is different in the coax between the radio and antenna when radios are switched?

I've seen this before when the coax, jumpers, wattmeter radio, power cables and power supply cables can interact a little with the SWR because its all connected to the shield of the coax and hot with RF. If this is the problem I would add a good 1:1 choke balun near the antenna and retune the antenna, then all variations of radios and coax should all read the same SWR.

Another potential problem could be one radio has a problem and its putting out lots of harmonics that are not far down from the main frequency. That would show up as some reflected power because it would be in the 50MHz range hitting an antenna in the 28Mhz range and reflecting back. Something would have to be really screwed up for that much harmonic energy to come out of a radio.

I used the same equipment and cables for both readings. I also had a friend test it on his vehicle and he got a 1.3 reading so I know that the radio is fine. However his antenna is permanently mounted to the vehicle while mine is a magnet mount. I will be rechecking all cables, ground wire etc to see if I missed anything when I checked it last. I really appreciate everyone's input.

 

[fantasma25]

All amateur radios I know of over the last 40yrs or so have band pass filters that switch in the right places for all frequencies from 1.6 to 30Mhz when the radio is modded for wide band operation. I have had four Icom 706 series radios and still run a 706MKIIG in my truck. It works beautifully out of band with no more harmonics than operating within the ham bands. That is by design as the radio mfrs know many of their radios will be used out of band.

The Icom 706MKIIG is not a $150 swap meet rig, they still fetch over $500 in many cases because its a very very good radio for mobile use and much better than its replacement the IC-7000 and many newer radios sold today. If and when it breaks (doubtful) I have a new IC-7100 to replace it and the 706 has about the same performance.

I got a pretty good deal on the radio which is why I bought it. The seller tested the radio in front of me and we measured power out, swr etc. All seemed to be fine. The person I bought it from is well known for the quality of his radios. I'm pretty sure it is something on my end, just trying to figure out what it is.

 

[prcguy]

This right here is the reason you are seeing changes. A mag mount uses the coax as part of the antenna and its sensitive to any changes like the radio connected to it, the length of its power cables, the length of its mic cable, anything that changes the electrical length of the ground system which is everything connected to the radio.

Its possible to isolate the mag mounts coax from the rest of the world using ferrite but you would end up retuning the antenna after the ferrite is in place. Your better off just using it the way it is and forget about any minor SWR changes.

However his antenna is permanently mounted to the vehicle while mine is a magnet mount.

 

[FiveFilter]

That's an interesting observation prcguy.

All I use is mobile CB since I only use a shortwave when traveling long distances in order to keep up with the truckers' reports on highway traffic and other situations. And since I use several vehicles, all I use is mag mounted antennas. They work really well, and I seem to communicate as well as anyone else out there.

I didn't think of the SWR factor that you brought up. Although I've always been able to achieve low SWR values with my mag mounts, I do remember the advice that when using a mag mount, it is particularly important to ground the radio case. I suppose it's because of the way the mag mount attaches to its base, ie the metal body ground of the vehicle on which it is placed, and the potential for common mode currents. So, I always attach a ground wire via alligator clips running from the radio case to a ground, which usually is a seat frame that is anchored to the car body.

I assume grounding the case of the radio (and any amplifier being used) helps negate any problems related to common mode currents with the mag mounts. It likewise should help negate any SWR changes seen when switching radios, as per the OP's situation, no?

 

[prcguy]

Every mag mount antenna and installation is different. Some mag mounts couple to the car body better than others and will be more stable. Others not so much and the coax is really sensitive to things around it. Some car or truck roofs are larger, providing a better ground plane and others are small with a poor ground plane at 27MHz. Grounding the radio to the car chassis changes things and in some cases it might be enough to stabilize the SWR and sometimes not.

RF does not automatically go to ground with your wire and alligator clip, its just another place it can divide and flow but it will usually make things better. Without the ground wire to the radio chassis there might be a lot of RF traveling down the coax to the radio through its power leads and mic cord to your hand, etc. When you ground the radio chassis with a separate wire the RF will divide between different paths so there will be less RF on other paths like the mic cable to your hand. This will depend some on the lengths of the different paths with some having more or less RF currents based on resonant lengths, etc.

In the 70s and 80s I installed a lot of radios and antennas, we figure about 1,000 or so. I would estimate 100 to 200 were mag mounts and I would get some that tuned up perfect, worked great and never had RF on the mic or SWR changes when you touched the coax, etc. Others were trouble and in many cases there was nothing obvious I could find between the ones with no trouble and the PIA installs.

Some people had a cigarette lighter plug and the radio sat ungrounded on a seat cushion and the mag mount antenna worked great. Some radios were bolted and well grounded in a metal dash of a big 70s vehicle and they gave me grief, every install had a different outcome. I don't remember which mag mount antennas worked better back then but I think some of the brands that had a tapped base coil might have been less trouble than just a hot loaded stick like a center load or Firestick. K40s were kind of new and expensive back then but had a huge magnet that coupled well to the car body and I sort of remember them having less floating RF problems.

I assume grounding the case of the radio (and any amplifier being used) helps negate any problems related to common mode currents with the mag mounts. It likewise should help negate any SWR changes seen when switching radios, as per the OP's situation, no?