IC-705 SWR meter not matching analyzer's values

[montyhouse]

I built a 6m dipole and tested it today (with Comet CAA500 analyzer). SWR readings from 50-54 MHz. None higher than 1.6.



I then connected my IC-705 to see if anything was going on the band and decided to test SWR. The figures from the radio were considerably higher, and I can't figure out why.

• Antenna ~ 10' in air (> 1/2 wave)
• Coiled choke near feed point
• Tuner off; max power 10w (per manual)
• Set radio to RTTY (per manual)

Any thoughts are appreciated.

 

[prcguy]

I see you mention a tuner. Was the tuner in line (bypassed) with the antenna analyzer when taking measurements? Was the tuner in line (bypassed) with the radio when taking measurements? If it was not in line with the antenna analyzer and it was in line with the radio there is the problem, anything in line like an antenna tuner will change the VSWR.

Measure with the antenna direct into the antenna analyzer and measure with the antenna direct into the radio, no tuner at all. Results should be similar and within whatever cal spec or accuracy each SWR meter is capable of.

 

[montyhouse]

I see you mention a tuner. Was the tuner in line (bypassed) with the antenna analyzer when taking measurements? Was the tuner in line (bypassed) with the radio when taking measurements? If it was not in line with the antenna analyzer and it was in line with the radio there is the problem, anything in line like an antenna tuner will change the VSWR.

Measure with the antenna direct into the antenna analyzer and measure with the antenna direct into the radio, no tuner at all. Results should be similar and within whatever cal spec or accuracy each SWR meter is capable of.

No, the feed from the antenna went through the disconnected tuner. But, assuming I run the coax directly to the radio and get a reading similar to what the analyzer gave me, wouldn't that question the radio's ability to get an accurate reading in the field per its instructions? (And, no, it's a Mat 705 tuner, not an Icom.)

 

[prcguy]

No, the feed from the antenna went through the disconnected tuner. But, assuming I run the coax directly to the radio and get a reading similar to what the analyzer gave me, wouldn't that question the radio's ability to get an accurate reading in the field per its instructions? (And, no, it's a Mat 705 tuner, not an Icom.)

If you have a particular VSWR on an antenna, if you run that through a tuner, even bypassed, it will change the VSWR. If you want an honest comparison of SWR meters you must plug the exact same thing into the antenna analyzer and into the radio. Repeat the test with no tuner connected, just the antenna feedline into the analyzer and radio and it that is the best you can do.

 

[prcguy]

No, the feed from the antenna went through the disconnected tuner. But, assuming I run the coax directly to the radio and get a reading similar to what the analyzer gave me, wouldn't that question the radio's ability to get an accurate reading in the field per its instructions? (And, no, it's a Mat 705 tuner, not an Icom.)

To add more, if the tuner is connected to the radio and bypassed and it messes up the match some which is typical of many bypassed tuners, the radio would be giving you an accurate reading in the field per its instructions. If you look inside your tuner the coax connector will sometimes have a long bare wire going to a circuit board trace and through relay contacts and many tuners have a common trace with a bunch of relays attached to that trace with their internal wiring up to the contacts that are not in use, so a bypassed tuner can be a lousy match going through it bypassed and its always worse on 6m.

I've had several tuners here that were so bad on 6m it wasn't worth having them in line. They made the match better at the cost of more insertion loss than the mismatch caused.

 

[montyhouse]

PRC, thanks for your help. Again.

It does, however, beg the question of why Icom says simply turning off the tuner (vs. removing the ATU's coax connection and, instead, running directly to the radio) will yield an accurate SWR reading. I'm using a MAT tuner vs. Icom ATU.... would it work with the latter but not the former?

 

[prcguy]

If when using the Icom tuner the radio is still using the SWR sensing in the radio, it will not always give an accurate measurement of the SWR at the antenna or tuner input. The tuner bypass path and the coax jumper between the radio and tuner will often change what the radio SWR meter reads. The only way that can work is if the tuner has accurate SWR sensing and feeds that to the radio display. I don't believe it does as Icom has used a four wire tuner interface for years that only supplies power, start and end tune type signals and no SWR readings.

As an example, if you have a 50 ohm resistive dummy load at the tuner input and the tuner bypass path is a very good 50 ohm path, then the radio should show an accurate SWR of the load on the tuner. Many types of antennas, especially end feds using a 9:1 balun rely on the coax as part of the antenna. Changing the length of the coax on these type antennas can change the SWR measured at the radio and so will the bypass path inside the tuner and the coax jumper between the tuner and the radio. The resulting SWR measured at the radio can be influenced by many things and I don't think Icom is taking this into account with their statement of turning off the tuner to get an accurate SWR reading of the antenna.

 

[BCBH]

I've seen something similar. Using an analyzer I get one SWR reading (1.6:1) after disconnecting the analyzer and connecting directly to the radio the 705 shows an SWR of <3:1. Puzzling.

Roger
N1ROG

 

[CaliHam]

I have similar issue. I would hookup my chameleon loop 2.0 to my Icom7300 just across the room tune the loop get good SWR. I would take the same antenna connect to Icom Ic705 SWR is high connect back to my 7300 and SWR is perfect right where I tuned it anyone is experiencing this.

 

[prcguy]

Try this, get two known good 50 ohm resistive dummy loads. Try one on the radio and one on the antenna analyzer, they should both read 1.0:1 or very close. Then connect the two loads to a T adapter to make a 25 ohm resistive load and check directly on the radio then antenna analyzer, it should read exactly 2.0:1.

That will tell you if the SWR meter in the radio and the antenna analyzer are calibrated in the lower VSWR ranges. If they are then any discrepancy you are seeing is probably from passing through the tuner while bypassed or any extra cables in line with one setup vs the other.

 

[k6cpo]

I've seen something similar. Using an analyzer I get one SWR reading (1.6:1) after disconnecting the analyzer and connecting directly to the radio the 705 shows an SWR of <3:1. Puzzling.

Roger
N1ROG

With that kind of disparity in readings I'd question the calibration of either the analyzer or the radio. And I'd be more likely to trust the analyzer rather than the radio.

 

[kh2]

Hate to dredge up a bad meal but has anyone found a solution or explanation for the discrepancy in SWR readings?? I'm running into the same thing. 1.5 on analyzer and infinity on 705.

No tuner is connected. Tried the dummy load and read a 1:1 at radio. I have not check 2x dummy loads.

My concern is, is this an indicator of something else wrong? or just an anomaly.

Has anyone contacted ICOM?

Thanks, Henry

 

[AB5ID]

Hate to dredge up a bad meal but has anyone found a solution or explanation for the discrepancy in SWR readings?? I'm running into the same thing. 1.5 on analyzer and infinity on 705.

No tuner is connected. Tried the dummy load and read a 1:1 at radio. I have not check 2x dummy loads.

My concern is, is this an indicator of something else wrong? or just an anomaly.

Has anyone contacted ICOM?

Thanks, Henry

Is the vswr reading on the icom consistent starting from the lowest power setting and slowly increasing or does it change. If it changes it may be RF feedback causing issues with the SWR reading. Similar to breaking into oscillation or having too much common mode feedback.

 

[kh2]

Same SWR regardless of power. I do have 5 coils at the antenna mount and ferrites inside at the feed - they are not calculated wraps/ferrites just a "swag".

Thanks for the interest it's a head scratcher

 

[montyhouse]

Hate to dredge up a bad meal but has anyone found a solution or explanation for the discrepancy in SWR readings?? I'm running into the same thing. 1.5 on analyzer and infinity on 705.

No tuner is connected. Tried the dummy load and read a 1:1 at radio. I have not check 2x dummy loads.

My concern is, is this an indicator of something else wrong? or just an anomaly.

Has anyone contacted ICOM?

Thanks, Henry

Yes. Icom tech support told me to remove ATU (MAT-705) which was connected and the problem went away.

 

[prcguy]

Any external tuner will have RF running through cables, circuit board traces and relay contacts. You will never have the same SWR reading going through a tuner as you would going right to he radio.

 

[kh2]

Again - No tuner is connected during the tests.

I did try radials 4ea 5'. Using the 705 SWR charting the reading is off the chart. under live test on transmit at 52 MHz it read ~ 9 still way high but lower than infinity. The Nano VNA read the same ~ 1:1.5ish

If it's an anomaly I will find it - every single time ;-)

 

[merlin]

It is not an anomaly, a Tuner inline will put a monkeywrentch into your SWR.
Testing the antenna witha calibrated Nano VNA, that is going to be your close SWR.
My Kenwood is not so different. I have to use an outboard ATU because the random wire antenna is too reactive for the internal ATU.
At a frequency, I turn the internal to bypass, tune the outbord (Icom AT-140) then look at the SWR and sometimes 2:1. (2.8:1)
I further tune with the intenal and that gets the SWR down below 1.5:1.
The OPs antenna looks pretty good. Suggestion, go straight to the radio, nothing inline. Bypass the internal tuner.

 

[merlin]

Is the vswr reading on the icom consistent starting from the lowest power setting and slowly increasing or does it change. If it changes it may be RF feedback causing issues with the SWR reading. Similar to breaking into oscillation or having too much common mode feedback.

Power level should have zero effect on SWR. Antenna analizers use typically 0 Dbm. Tune mode on tranceivers may be from 5 to 25 watts.

 

[merlin]

Is the vswr reading on the icom consistent starting from the lowest power setting and slowly increasing or does it change. If it changes it may be RF feedback causing issues with the SWR reading. Similar to breaking into oscillation or having too much common mode feedback.

OP indicates he is using a CM choke at the antenna. With a tuned dipole, it should be little or no effect.