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Author Topic: Daiwa or MFJ  (Read 2198 times)
W6AOA
Member

Posts: 66




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« on: January 15, 2017, 11:38:55 AM »

I have two pieces of equipment that show very different measurements of a 2 meter Halo antenna (mike Fedler N6TWW
on youtube)

FREQ:      144.350

MFJ-266   says       SWR 1.1    54+j   10   51

Daiwa CN-801        SWR  1.75


I would like some opinions of what I should believe...

Thanks
Ed  W6AOA Pensacola, FL
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W9IQ
Member

Posts: 1644




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« Reply #1 on: January 15, 2017, 12:12:48 PM »

Halo style antennas often cause common mode current on the coax. If common mode current is present, the feedline is participating as part of the antenna. If you then alter the effective length of the feedline, you alter the antenna tuning.

While you didn't specify your test setup, I would imagine that the total length of coax you used changed when you used the MFJ compared to using the Daiwa since the Daiwa also needs a transmitter to work. This could account for a significant difference in measured SWR if common mode current is present. Adding a VHF choking balun to the coax as it exits the antenna may help to mitigate this effect.

Sometimes you can see the effect of common mode current by sliding your hand along the length of the coax while taking your SWR reading (without getting too close to the antenna). If it changes at all, then you have common mode current. This is not a foolproof test but it is often quite observable.

Another thing to think about is that the Daiwa is rated +/- 10% full scale accuracy (and this is probably questionable across all bands). Because the Daiwa is a cross needle type meter, this error occurs separately in the forward and reflected readings. This can account for a significant departure in the SWR reading compared to actual.

Also consider that the MFJ takes its SWR reading using very low power while you probably applied a few watts using the Daiwa. Sometimes connection and coax problems don't show up until exposed to higher power.

If you have a quality VHF dummy load, you may wish to repeat your tests but substituting the dummy load for the antenna using all of the same coax, etc. This may pinpoint any equipment or procedural problems.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W6AOA
Member

Posts: 66




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« Reply #2 on: January 15, 2017, 01:21:53 PM »

Thanks Glenn,

I hv a five turn 8 inch loop coax choke at the antenna.    I do have a vhf/uhf dummy load and will see if the coax is at fault.   Im using only 20 watts from the transceiver  with the daiwa.   By the way the daiwa and the MFJ266 are/is connected "inside the shack",  I've  done other antenna tests  with the swr meter AT the feedpoint and almost always get something different like that.  Thanks again.. Ed W6AOA
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W8JX
Member

Posts: 12080




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« Reply #3 on: January 15, 2017, 06:23:50 PM »

I have two pieces of equipment that show very different measurements of a 2 meter Halo antenna (mike Fedler N6TWW
on youtube)

FREQ:      144.350

MFJ-266   says       SWR 1.1    54+j   10   51

Daiwa CN-801        SWR  1.75


I would like some opinions of what I should believe...

Thanks
Ed  W6AOA Pensacola, FL


Common mode current aside I would trust a Daiwa over MFJ any day...
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Ham since 1969....  Old School 20wpm REAL Extra Class..
WB2WIK
Member

Posts: 21749




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« Reply #4 on: January 17, 2017, 11:22:41 AM »

Thanks Glenn,

I hv a five turn 8 inch loop coax choke at the antenna.    I do have a vhf/uhf dummy load and will see if the coax is at fault.   Im using only 20 watts from the transceiver  with the daiwa.   By the way the daiwa and the MFJ266 are/is connected "inside the shack",  I've  done other antenna tests  with the swr meter AT the feedpoint and almost always get something different like that.  Thanks again.. Ed W6AOA


A five turn, 8" diameter coax choke doesn't do anything at 50 MHz.    Where did those dimensions come from?

The Daiwa needs a transmitted signal to indicate anything, so when you measure using that, you have a different "antenna system" than just using a hand-held antenna analyzer which has no connection to a transmitter.   I'd expect them to read differently unless the termination is perfect.

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WB6BYU
Member

Posts: 16877




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« Reply #5 on: January 17, 2017, 02:44:19 PM »

G3TXQ's table of common-mode chokes shows that a 5-turn air core choke 7" in diameter is
effective between 12 and 22 MHz.  Above that frequency, and especially on 2m, the capacitance
between the turns allows RF to bypass the coil.

One way to check for possible common mode current is to touch one end of an 18" wire to the
coax shell on the MFJ analyzer while taking a reading and see if the SWR changes.

Having common mode current doesn't mean that the antenna won't work, but rather that
performance (pattern, SWR, etc.) may vary unpredictably with such things as coax length,
how the radio is grounded, and even plugging in a set of earphones.
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