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KF5CQJ
08-22-2009, 09:55 PM
HI to all,
I just finished checking my SWR and OHm levels for my 2 meter New Motorola NMO thru the roof mount. I got a 1.8 SWR and a 90 ohm's load on the MFJ 259b @ 146.250. Isn't the Ohm's kinda high or is this normal for the NMO thru the roof mounts? I had Comm South install the NMO on my truck cab two days ago, Is not the ohm's suppose to be 50 ohm's? Please help me on this, my 2 meter is getting out and averaging about 40 miles radius with the 1/4 wave 19" whip.
73's
Albert KF5CQJ

WB2WIK
08-22-2009, 10:25 PM
HI to all,
I just finished checking my SWR and OHm levels for my 2 meter New Motorola NMO thru the roof mount. I got a 1.8 SWR and a 90 ohm's load on the MFJ 259b @ 146.250. Isn't the Ohm's kinda high or is this normal for the NMO thru the roof mounts? I had Comm South install the NMO on my truck cab two days ago, Is not the ohm's suppose to be 50 ohm's? Please help me on this, my 2 meter is getting out and averaging about 40 miles radius with the 1/4 wave 19" whip.
73's
Albert KF5CQJ

Which whip are you using, exactly? The impedance has nothing to do with the NMO mount, it has everything to do with the whip you screw into it.

K7MH
08-22-2009, 10:40 PM
Never mind..I overlooked something that was stated.

K8JD
08-23-2009, 12:01 AM
If your MFJ tester says 90 ohm Z across the whole band, it's not too bad. A quarterwave on a flat plane ground like a car roof isn't really quite 50 ohms. Actually I would expecct it to be lower. I think you may be using something other than a simple quarterwave (20 inch) whip ant now...I just skimmed over the other answers :D
Most Motorola antennas are based on the 150-175 Mhz commercial band. Mab be a bit short for 2M.

N5RFX
08-23-2009, 12:18 AM
50 * 1.8 = 90

one relationship of swr is the load impedance/source impedance

73,
Mark N5RFX

K9ZMD
08-23-2009, 01:03 AM
When you connected your MFJ 259B, did you happen to notice what frequency gave you the lowest SWR reading? I'm guessing you didn't, or you would have had no question, or asked a slightly different question here.

Note: You do not have to keep the 259B frequency within the ham band; slowly sweep above & below the 2 meter band until you find the lowest SWR. Now, is that frequncy higher than your intended center frequency, or lower?

If lower, you need to shorten the whip length to get less than 1.8:1 SWR at 146.250 MHz. If it was a higher frequency, you'll need lengthen the whip for a quarter wave at the target frequency. With some antennas, the whip mount allows you to slide the whip in or out a bit to change its length. Sometimes, though, you've got to do some cutting.

If that's the case, here's a tip. I usually install a piece of coat hanger wire in place of the original whip, then find the correct length with a few easy cuts on mild steel. Once I know the correct length, my first cut on the stainless steel whip should also be my last one.

Once you have the 1/4 wave whip length adjusted, don't worry about the feedpoint impedance. It will be lower than 50 ohms, your SWR will be less than 1.5:1, and your antenna will work about as good as you could hope for. 73

Gary, K9ZMD/6

KF5CQJ
08-23-2009, 03:56 AM
If your MFJ tester says 90 ohm Z across the whole band, it's not too bad. A quarterwave on a flat plane ground like a car roof isn't really quite 50 ohms. Actually I would expecct it to be lower. I think you may be using something other than a simple quarterwave (20 inch) whip ant now...I just skimmed over the other answers :D
Most Motorola antennas are based on the 150-175 Mhz commercial band. Mab be a bit short for 2M.
Thank you all for the advice!
I was thinking the same thing, that Comm South use a commercial band whip for my NMO mount, all the 1/4 wave's I seen on ham store's are longer and some are base loaded. I will go ahead and order a Comet or Larson 2 meter stick. Then I will retest the feedpoint again.
Thanks and 73's,
Albert KF5CQJ

KF5CQJ
08-23-2009, 04:03 AM
When you connected your MFJ 259B, did you happen to notice what frequency gave you the lowest SWR reading? I'm guessing you didn't, or you would have had no question, or asked a slightly different question here.

Note: You do not have to keep the 259B frequency within the ham band; slowly sweep above & below the 2 meter band until you find the lowest SWR. Now, is that frequncy higher than your intended center frequency, or lower?

If lower, you need to shorten the whip length to get less than 1.8:1 SWR at 146.250 MHz. If it was a higher frequency, you'll need lengthen the whip for a quarter wave at the target frequency. With some antennas, the whip mount allows you to slide the whip in or out a bit to change its length. Sometimes, though, you've got to do some cutting.

If that's the case, here's a tip. I usually install a piece of coat hanger wire in place of the original whip, then find the correct length with a few easy cuts on mild steel. Once I know the correct length, my first cut on the stainless steel whip should also be my last one.

Once you have the 1/4 wave whip length adjusted, don't worry about the feedpoint impedance. It will be lower than 50 ohms, your SWR will be less than 1.5:1, and your antenna will work about as good as you could hope for. 73

Gary, K9ZMD/6
Gary,
Thanks for the advice and the new idea of using a cloth hanger to get the correct length before cutting the stinger! I didn't sweep the band because I was letting another guy do the test. I just seen the results on the meter when he got it to the frequency. I should have a analyzer by next month and I will do the sweep with the new NMO 2 meter I buy. Thanks for the insight of troubleshooting!
73's
Albert KF5CQJ

KA9MOT
08-23-2009, 04:04 AM
Me thinks he missed the point......

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