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Author Topic: Icom versus MFJ conflicts  (Read 292 times)

Posts: 16

« on: July 28, 2009, 09:29:52 AM »

Hello all.

Where should I start to look for an explanation as to why I can dial up  resonant readings on various bands with an MFJ 259 analyzer, but when I connect the same coax to the same antenna,in the same location,elevation, etc, my 706MKiiG says "not so fast, pal", and renders high SWR indications? The antenna under test was a commercial

I don't think the radio or MFJ are mis-reading, as a connection to a Par end-fed 20M wire, and also to a VHF-UHF antenna yields expected read-outs from both the analyzer and my 706.

I'm not hugely technical in this field, but it is uber-frustrating.
Thanks for any trouble-shooting suggestions.

Posts: 35

« Reply #1 on: July 28, 2009, 09:58:35 AM »

Blair, First a few questions,
1. Sounds like you are using the 259 at the station end of the system and not nearer the antenna. Is that correct? Are you using the same frequency?
2. Have you checked the calibration of the analyzer. Connected to a 50 ohm load, it should read 1:1. 75 ohm,
2:1. Also is it a later version that shows Xr and Xc?.
3. Have you checked it with a different length of coax?
4. Please describe the system. Frequency, antenna length and height, feedline (type, length), are you
using a matching unit?

My first thought is that if its an early version of the 259, there could a complex load on the on the system and this could be the reason the Icom is not work performing. The length of the feedline can transform the impedance, depending on length and frequency. My favorite comment is "Its a system. Antenna, ground, feedline, tuner and radio". Awaiting your answer. 73s Don NJ2E

Posts: 5385

« Reply #2 on: July 28, 2009, 10:45:48 AM »

"Resonant readings" would mean that the capacitive reactance equals the inductive reactance, but does not give us the radiation resistance of the antenna.
The SWR shows the ratio of impedance to 50 ohms, but does not give us the reactances of the impedance.
Note that your MFJ259 has a range limit of 0---1000 ohms  resistive and 0---600 ohms reactive, but works quite well in normal amateur usage.
A SWR of 1:1 does not indicate that you are at a resonant point.


Posts: 9800


« Reply #3 on: July 29, 2009, 06:52:45 AM »

This is not meant to be condescending. That said, get out the manual for the 259, and read it! The questions you ask are indeed answered therein.

Alan, KØBG


Posts: 12764

« Reply #4 on: July 29, 2009, 12:30:55 PM »

WA3SKN wrote:
"A SWR of 1:1 does not indicate that you are at a resonant point."

If the impedance of your coax is exactly 50+j0 ohms and
the SWR is 1 : 1, then it certainly IS resonant.  The SWR
can only be perfect when the load impedance exactly matches
the characteristic impedance, so this would imply that the
load impedance is also 50+j0, therefore resonant since
j = 0.

In practice, of course, the characteristic impedance of
feedlines is slightly reactive due to the losses:  for
RG-213 at 10MHz the characteristic impedance is 50-j0.33 ohms.
So if you want to get very precise it isn't EXACTLY resonant,
but close enough for most purposes.

But back to the original question:  if the SWR analyzer
shows a low SWR (which is more important than exact
resonance) and the rig reports a different SWR, there
are a few possibilities:

(1) One or the other SWR indicators is not calibrated
correctly.  This happens all the time.  Check them both
on a dummy load, and also on a 2 : 1 load made by
connecting two dummy loads in parallel.

(2) One or the other signal sources has high spurious
outputs, often harmonics.  While a dummy load will show
a low SWR at all frequencies, few antennas will do the
same.  For example, if you are tuning on 20m and your
radio has significant harmonic output on 10m, there
will be a lot of reflected power on the harmonic which
will cause the SWR to read higher than with a pure 20m

(3) If you are swapping between the rig and the SWR
analyzer at the shack end of the coax and there is a
difference, it may be due to the difference in grounding
between the two.  The rig chassis should be grounded, at
least through the third wire of your electrical system.
If that ground system becomes part of the antenna when
the rig is connected but not when you use the analyzer,
you are measuring two different antenna systems, so there
is no reason to expect the same reading on each.  One
quick way to check this is to connect a clip lead from
the ground side of the analyzer to the rig ground and
see if that changes the analyzer reading.  If this is
your problem, you may want to try a balun / feedline
choke to reduce the effect of the grounding on SWR.

Posts: 16

« Reply #5 on: August 24, 2009, 03:10:00 PM »

Way back in the Jurassic Period, (July 28) I posted a question about why I was getting different readings from my 706MKIIG and an MFa J analyzer, looking for the sweet spot on various bands of a commercial Buddipole.
 Shortly after, I put the BP away and haven't re-visited it. Besides the analyzer had to be returned to my ARC.

Just a belated thanks to all who sent along suggestions to try with conjecture as to what was going on. (Some of them didn't penetrate my thick skull, but I will save the postings and get the 259 again and have another go at the Buddipole before the rainy season descends as surely it will in the Pacific Rainforest.
Blair VE7HHH.

Posts: 12337

« Reply #6 on: August 24, 2009, 04:24:09 PM »

Par end-fed 20M wire
This antenna uses the coax shield for the counterpoise. That means you have RF flowing on the outside of the shield. Depending on the length of the coax and where/how it is run you may have RF in the shack affecting the Icom. Try a feed line choke just before the coax enters the shack.
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