Why is my antenna analyzer lying to me?

[VA3WEX]

I discovered a wierd phenomenon, that perhaps someone can explain.

Why is it than an antenna analyzer can seemingly tell you the SWR is going to be, say, 1.3. But when you put the radio in there instead of the analyzer, the SWR can be wildly different.

This afternoon I was doing just that with an OPEK HVT-600 antenna. I had the telescopic section out to a certain distance, and the analyzer told me the SWR was going to be about 1.5 at a frequency of 7.250 MHz. But when I put the radio on the end of the coax instead of the analyzer, at the same 7.250 MHz, the SWR was too high for the tuner to handle--so, something over 5. Nothing whatsoever changed with the setup: just unplugging the analyzer and plugging in the radio.

Is the analyzer just out of whack and needs calibrating somehow? (I have no idea how to do that...)

Or is it something related to the power level? The radio is only a 6-watt FT-818, but the analyzer is putting out only fractions of that wattage. But if a change in wattage makes so much of a difference, how can one ever "trust" an antenna analyzer?

 

[prcguy]

The OPEK antenna is a tiny HF antenna with no ground plane or counterpoise, it will be greatly influenced by the radio and your body. When you checked it you had an antenna analyzer probably held in your hand. Was the radio also in your hand or was it sitting on something insulated? The mic cable on the radio will become part of the antenna counterpoise and will change the match. Was the radio operating on batteries or from an external power source? The power cables to an external power source is part of the counterpoise and will change the tuning, etc.

 

[VA3WEX]

Antenna was on magmount on roof of my car. The analyzer was on my lap, leaning against the steering wheel: the radio was on the dash in front of the steering wheel. The coax came in through the rear passenger window, over the passenger seat, over the mirror then to the device in question--a slight change of position because of the slight change of position of the devices. In both cases, all doors were closed. Radio operating from cigarette jack (engine off). Analyzer on batteries.

 

[prcguy]

The radio connected to the cig jack can skew things in your case. That type antenna on a mag mount will leave the coax hot with RF and minor changes at the radio end will change the VSWR like the difference between a meter in your lap vs a radio with a ground lead connecting the coax to the vehicle chassis. If you were to run the radio off batteries from your lap similar to the meter then the results would be closer.

Most mag mounts have very insufficient coupling to the vehicle body at HF frequencies and will always light up the coax with RF. I fixed that by getting an 8 1/2" X 11" sheet of thin refrigerator magnet material and coating it with thin copper sheet, then a short wide braid to my HF mag mount. That changed my mag mount HF antenna from something similar to yours with unpredictable and changing VSWR to a very stable VSWR and basically the same as using a mount grounded to the vehicle body. I also used a 1:1 ferrite choke balun at the radio end to make sure there was no hot RF still riding on the coax. With that mag mount I can go from an antenna analyzer to a grounded 100w radio or a QRP rig sitting in my lap and the VSWR is the same in all cases.

Diamond sells a magnetic ground enhancer that is about 5X smaller than the one I made and they have much too long of a ground wire and they charge way to much. You can make one like the picture below for a lot less and its really effective.

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Antenna was on magmount on roof of my car. The analyzer was on my lap, leaning against the steering wheel: the radio was on the dash in front of the steering wheel. The coax came in through the rear passenger window, over the passenger seat, over the mirror then to the device in question--a slight change of position because of the slight change of position of the devices. In both cases, all doors were closed. Radio operating from cigarette jack (engine off). Analyzer on batteries.

 

[VA3WEX]

Excellent and useful--thank you very much!

 

[Ubbe]

There's a distance between the metal in the magnet mount and the metal of the car, both paint and perhaps rubber. The distance equal a capacitor of very low pF value, so ground will work better at VHF/UHF frequencies than at HF.

/Ubbe

 

[slowmover]

Looking at this idea to try all along the windshield base of a Freightliner Cascadia from door-mount antenna bases.

Post in thread 'Uniden bearcat 980 insane swr.'
http://forums.radioreference.com/threads/uniden-bearcat-980-insane-swr.449813/post-3825570

Thx for the idea!!
.

 

[AB5ID]

You could have a lot of strong common mode currents that really won't show up unless you're using some RF power. The common mode currents can create havoc with feedback self oscillation and RFI.

 

[buddrousa]

The quality of the antenna analyzer could also play a difference.

 

[merlin]

The analyzer is likely not lying unless it is a piece of junk.
The SWR sensing is also very different in radios.
To properly evaluate an antenna between the two, there can be no changes at all with the antenna and feed.
No changes to frequencies, only the swap from analyzer to radio.
No repositioning of the antenna.
Nearby conductors can have some effects. Why antenna manufacturers use a test range to evauate an antenna.

 

[buddrousa]

Today quite a few people think that a $50 device from China is the same as a name brand device. A prime example is someone that thinks the cheap Chinese DMR radios are the same as Motorola radios.