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

[fantasma25]

It was my understanding that once a cb antenna was tuned that any radio could be used. I have an HR2510 and when connected to an external meter I get a low reading (1.2-1.5). Now without moving my antenna (Wilson 1000 - Magnet mount), I switch the 2510 for an Icom706 and measure the swr using the same frequency but I now end up with a higher SWR reading (2-3). What would cause such a big difference? I even lowered the power on Icom to it's lowest setting to see if that would make a difference but it did not. Any thoughts?

 

[robertmac]

Icom 706 as a CB rig?

 

[ka3aaa]

component tolerances will cause that to happen. A manufacture could make a million radios at the same time and every one will be slightly different somehow.

 

[bill4long]

It was my understanding that once a cb antenna was tuned that any radio could be used. I have an HR2510 and when connected to an external meter I get a low reading (1.2-1.5). Now without moving my antenna (Wilson 1000 - Magnet mount), I switch the 2510 for an Icom706 and measure the swr using the same frequency but I now end up with a higher SWR reading (2-3). What would cause such a big difference? I even lowered the power on Icom to it's lowest setting to see if that would make a difference but it did not. Any thoughts?

Using the same SWR meter both times?

 

[romanr]

If the antenna, coax, connector(s), and meter are the same, the best explanation is that the radios are transmitting on different frequencies.

Are you using an adapter on one radio and not the other?

 

[mmckenna]

What brand/model SWR meter?

Are you using the same frequencies each time?

Are you recalibrating the meter each time?

 

[jonwienke]

You have to readjust the meter between radios to account for the difference in TX power.

And if you're switching frequencies, all bets are off.

 

[romanr]

component tolerances will cause that to happen. A manufacture could make a million radios at the same time and every one will be slightly different somehow.

I agree the slight differences can be expected due to manufacturing tolerances and component variations, but there is a big difference between SWR of 1.2 and 3.0.

SWR is a simplistic measure of the ratio of power going towards an antenna vs power reflected back, so radio variations shouldn't have this much impact on the ratio UNLESS there two transmitters are on different frequencies. The antenna load is a complex impedance - it changes as the frequency changes. To see this much change, the most likely cause is different excitation frequencies. I would guess that the icom706 is transmitting at a different frequency, possibly due to being configured for a repeater split.

All of this assumes that you are using the meter correctly.

 

[ko6jw_2]

Aside from the legality of modifying a 706 for CB (it can be done apparently). I have found this radio to be sensitive to operating voltage and to grounding. I used one mobile for several years and had to use a voltage stabilizer as the radio did not like anything but 13.8 volts plus or minus a little. Also I originally used a trunk lip mount but had erratic performance. I used direct connection to the battery, but also added a ground strap from the ground terminal on the radio to the car chassis. All of this is good practice of course, but the 706 was more finicky the other radios.

 

[fantasma25]

What brand/model SWR meter?

Are you using the same frequencies each time?

Are you recalibrating the meter each time?

Same frequency, same power. The meter is a astatic pdc1

 

[fantasma25]

You have to readjust the meter between radios to account for the difference in TX power.

And if you're switching frequencies, all bets are off.

Correct, everything is remaining the same, even the output power.

 

[fantasma25]

If the antenna, coax, connector(s), and meter are the same, the best explanation is that the radios are transmitting on different frequencies.

Are you using an adapter on one radio and not the other?

No

 

[fantasma25]

Using the same SWR meter both times?

Correct, same meter.

 

[fantasma25]

I agree the slight differences can be expected due to manufacturing tolerances and component variations, but there is a big difference between SWR of 1.2 and 3.0.

SWR is a simplistic measure of the ratio of power going towards an antenna vs power reflected back, so radio variations shouldn't have this much impact on the ratio UNLESS there two transmitters are on different frequencies. The antenna load is a complex impedance - it changes as the frequency changes. To see this much change, the most likely cause is different excitation frequencies. I would guess that the icom706 is transmitting at a different frequency, possibly due to being configured for a repeater split.

All of this assumes that you are using the meter correctly.

Thank you, I will take a look just to make sure. I don't think it is configure for split operation but will check.

 

[buddrousa]

To make the answer short all amplifiers see loads different and have to be tuned or matched to the load.

 

[bill4long]

Correct, same meter.

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.

 

[bill4long]

To make the answer short all amplifiers see loads different and have to be tuned or matched to the load.

Both radios have solid state "final" power amplifiers and shouldn't be a factor in what he's seeing. The Icom is an old 706. Maybe it has a failure of some sort which is why I'd like to see the test done with a dummy load on the antenna side of the meter.

 

[buddrousa]

And this is why I can take 2 new Kenwood NX5800's and see different readings on the same antenna?

 

[prcguy]

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.

 

[mm]

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.