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[KI4DSC]

 I am thinking about installing an off center fed dipole so I can run on 80, 40, 20 and 10m without a tuner. But before I do that I wanted to map the SWR of my existing antenna.  My current antenna is a dipole, however it is not straight rather it is bent at about 140° forming a sort of  horizontal V.  This antenna is cut for 3.8 MHz and works well on the 80 m band without a tuner.  I was curious to see what the SWR was on all of the other HF bands. I hooked up my antenna analyzer and started sweeping the frequencies. I was very surprised when I found the highest SWR reading was 3.1 to 1 as I swept across all the bands from 80 to 10m.  I knew this was too good to be true, so I hooked up my radio, bypassing the tuner, and set the radio to 5 W. I spot checked many frequencies throughout the bands. As you may expect I found  much higher SWR readings when I use my radio. The readings I got with the radio on some bands were between 3:1 and 5:1.  Some areas of the bands were well over 6:1 and many areas exceeded the limits of my SWR meter.

 I tested my antenna analyzer using known value resistors on the end of a 25 foot lead of coacts. The results were as expected given the pure resistive load. I also added some inductance and repeated the testing and again I got the values I expected  to see.

 I'm at a loss, as the antenna analyzer appears to be functioning properly however when used on my current antenna the values are obviously incorrect.

 My current antenna is relatively low at only 26 feet above the ground. It's also quite near trees in some areas. I thought perhaps this was causing the difference in the SWR readings given different power settings. So after running the radio at 5 W I turned it up to 100 W and got essentially the same result.  Just to be sure I then turned on my amplifier and tried on one or two frequencies using the lowest setting (About 300 W) the SWR was slightly higher as expected but not significantly.

 Now I feel like I can't trust my antenna analyzer. I would love to hear some words of wisdom or information from someone who has run into this phenomenon and may know what is causing it.

[WB6BYU]

Do you have an effective balun on your antenna?

Such symptoms are often due to common mode currents on the
outside of the coax.  In such a case, the outside of the coax -
and everything connected to it - are part of the antenna.

When you swap from a hand-held analyzer to an HF rig that is
connected to the AC wire ground, for example, the SWR changes
because you have changed the antenna - it is now connected
to ground where it wasn't before.

Both meters may be giving the proper result, but they are
measuring different antennas!

Try this:  connect the analyzer to the antenna, check the SWR,
then temporarily connect the coax shield to the radio chassis
with a clip lead. If the SWR changes, you need a better balun.

[N3DT]

Don't know what your analyzer is, but if the step size is too large it may not be finding the high swr spots. You may need to reduce your scan range to smaller chunks and do more of them to cover the HF range. If it's a constantly varying range analyzer, I haven't a clue what your problem is.

[KI4DSC]

Well I think I may have solved it.  As I move higher in frequency the difference between my antenna analyzer and radio becomes less.  As I get up to 10m the difference is less than 1 and when I get to 6 meter everything begins to line up again. 

I have my analyzer grounded to my station ground.  I checked for common mode and I find no evidence of that, but just to be sure I changed my 1:1 bal-un for my 4:1 bal-un (both are ferrite type high power units) and other than the numbers changing due to the transforming action of the 4:1, the symptoms remained.

This is my theory; my antenna is supported at the center by the peak of my 2 story home. About 10 feet below is an aluminum screen enclosure.  My theory is that on the higher bands my antenna is interacting with that aluminum.  As I sweep into the 10 meter and 6 meter bands, the aluminum enclosure is far enough away that it no longer interacts.  As the power goes from near none with the analyzer to a real world 5, 10 or 100 watts things change with the way the enclosure interacts.

I have not moved the antenna to check my theory so it remains just that, a theory. 

I will be taking this antenna down and replacing it with an off center fed dipole to work 40, 20, 10 and 6 meter.  I also have a 6BTV 6 band vertical antenna that I use on 80 and 40 meter mostly. 

I will follow up with my results once the OCF dipole is up.  I have a suspicion that my problems will be solved - all be it I will certainly encounter new problems...

[KC1BMD]

How does your theory explain the difference between the rig meter and handheld analyzer? Did you try what BYU suggested?

[KI4DSC]

Yes.  I tried it and there was no change in the meter.  Like I said, my antenna analyzer is grounded to the station ground but I did try it just to be sure and it made no change. 

The only other variable is the power output - or my analyzer is broke... but it gives true values on my 6 band vertical.  Even if I put a 6:1 transformer in line to really blow the SWR on my vertical, my analyzer and radio agree... That is why I am so baffled as to what is going on with this dipole...  It is strange. 

[JS6TMW]

I'm only mentioning this now because I just discovered a measurement problem was due to a faulty F-to-BNC adapter....  check all your connectors and coax adaptors for integrity (wiggle the hell out of them).

Steve in OKinawa

[KI4DSC]

Thanks Steve... I will do that...

[KC1BMD]

What analyzer are you using? If possible, try borrowing another analyzer to see if any difference.

[KH6AQ]

Is your antenna analyzer an MJF-259B?

[KI4DSC]

Ok... I think I have found my problem... I am using the MFJ-259B.....  I know, I know, MFJ, but there is more to the problem...  I ran a test on the analyzer and this is what I found:

Using high quality non-inductive resisters---

Resistor | SWR |  Expected SWR |
50ohm  | 1:1   | 1:1 Good So far
100ohm| 2:1   | 2:1 what do you know.. looks good
150ohm| 3:1   | 3:1 looks like it is working so far...
200ohm|3.6:1 | 4:1 that's a little low... hmmm
300ohm|3.8:1 | 6:1 Oh this is getting bad now
550ohm|3.8:1 |11:1  This thing is way off...
1100ohm | 3.8| 22:1  Ok I give up!  this thing is obviously junk!

So with that being said, can this thing be repaired or should I just buy a new one.  I think I got this one about 5 or 6, maybe 7 years ago...

I tried it with both the batteries and with the external 12 volt plug and I got the same results.  

[KI4DSC]

On a better note, I got my Off Center Fed Dipole up!  I did not have room for the 7 band design because it is about 130 feet long and my home owners association (and the YL) will not allow the wire to be visible from the front.  So I had to go with the 4 band design that is 68 feet long.  It loads up under 3:1 SWR 40, 20, 10 and 6.  I can feed it with coax and not worry about feed line loss like I had with the 80 meter V dipole. The whole shootin' match is up just about 30 feet...

[KI4DSC]

Ok... I think I have found my problem... I am using the MFJ-259B.....  I know, I know, MFJ, but there is more to the problem...  I ran a test on the analyzer and this is what I found:

Using high quality non-inductive resisters---

Resistor | SWR |  Expected SWR |
50ohm  | 1:1   | 1:1 Good So far
100ohm| 2:1   | 2:1 what do you know.. looks good
150ohm| 3:1   | 3:1 looks like it is working so far...
200ohm|3.6:1 | 4:1 that's a little low... hmmm
300ohm|3.8:1 | 6:1 Oh this is getting bad now
550ohm|3.8:1 |11:1  This thing is way off...
1100ohm | 3.8| 22:1  Ok I give up!  this thing is obviously junk!

So with that being said, can this thing be repaired or should I just buy a new one.  I think I got this one about 5 or 6, maybe 7 years ago...

I tried it with both the batteries and with the external 12 volt plug and I got the same results.  

So after doing some research I have discovered my analyzer has likely suffered from a voltage input of more than 3 volts to the antenna port.  I have been careless and I have hooked the analyzer to long wire antennas without discharging the static to ground.  From everything I have read, it would appear I may have blown one or more of the detector diodes and now the unit can not properly function.  The question now is do I buy a better Analyzer or do I have this one repaired?

[G4AON]

Almost all antenna analysers are very easily damaged by static. However, your MFJ unit looks OK at close to 50 Ohms. There is an ARRL review of the similar 269 in the May 2005 QST, it shows reasonable accuracy, 5 Ohms (10:1 SWR) gave between 9:1 and 12:1 depending on frequency. 1000 Ohms (20:1) gave between 19:1 and 27:1. The measurements were on HF and 6m.

It is nowhere near the accuracy of my RigExpert AA-230 Zoom (review in March 2016 QST), but as it looks OK up to 3:1 on high Z loads, try it on 5 Ohms (3 x 15 Ohms spread around a plug). It might not be faulty, just not much use on very poor loads. Do test it with your resistive loads connected directly to the analyser, not on the end of 25 foot of coax, and see if that gives closer results.

73 Dave

[KH6AQ]

The MFJ-259 is affected by strong AM broadcast band signals.

One way to tell if this is the problem is to take measurements during the day and at night. Most AM BC stations substantially reduce power at night.