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Author Topic: Mobile 2 Meter 1/4 Wave Whips Have High SWR but 5/8 Checks Fine?  (Read 9014 times)
G7MRV
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« Reply #30 on: April 23, 2012, 01:07:11 PM »

Quote
You have to be very careful using the S-meter to make comparisons,

Exactly why i stated it was a Qualatitive test only, and gave no figures in dB


Quote
I don't mind the height etc. although I do have to be careful with these antennas on the van's high roof.

I would consider this more of a consideration than the performance difference. The 5/8th is considerably longer, and so is far more likey to get snagged in overhanging foliage etc. Ive lost the ends off several of these antennas due to very low bridges!
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MDNITERDER
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« Reply #31 on: April 24, 2012, 01:11:30 PM »

 Just some .02c I have the SWR Analyzer from MFJ-269 and nothing will read below 1.3-1.5 1/4 or even 5/8 wave.

 Now a Diamond SWR meter reads 1.2 so it appears thats the best the analyzer can do maybe.
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G7MRV
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« Reply #32 on: April 24, 2012, 01:24:54 PM »

Just some .02c I have the SWR Analyzer from MFJ-269 and nothing will read below 1.3-1.5 1/4 or even 5/8 wave.

 Now a Diamond SWR meter reads 1.2 so it appears thats the best the analyzer can do maybe.

What about a dummy load connected directly to the analyser? Before testing antennas on my car today, i proved the analyser into my Bird 500w load (only becasue it happened to be to hand, i'd normally have used the little 15w one!) X=0, R=50 to 52, SWR 1:1 from 1.8 - 170MHz.

I would never expect amateur equipment to be within 10% accuracy, probably barely 20%, unless calibrated against lab grade equipment.
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MDNITERDER
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« Reply #33 on: April 24, 2012, 01:41:16 PM »

 Actually I have not gone that far as I said the MFJ was not to far off from the SWR meter so i didn't sweat it. I think the only time I have never seen an SWR Meter move was on my 1/4 wave CB Antenna, "and I got tired of the up and down adjustments" Not saying a 1.1:1 is impossible on my current 1/4 wave and if my meter says 1.2 or 1.3 I am not gonna cry. Smiley

 Btw I did purchase 210 10Kohm 1W 5% Carbon film resistors to make a nice little dummy load.  Grin
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G7MRV
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« Reply #34 on: April 24, 2012, 01:58:00 PM »

Im with you on that, its not worth further hassle.
A normal, properly installed and working 1/4w should read about 1.3-1.5 anyway. Bear in mind as well that unless your coax is an exact 1/2w or multiple thereoff at the frequency in use, it will cause some impedence transformation and throw the numbers off a little. But 1.5 or lower is fine, your radio isnt going to be bothered.

Also bear in mind that the longer the coax run, and the worse its condition, the better the SWR gets! On some of the stuff i do commercially now, its only when the incoming MER gets low and the constellation plots resemble buckshot, that we find the coax has transformed into garden hose!

Btw I did purchase 210 10Kohm 1W 5% Carbon film resistors to make a nice little dummy load.  Grin

Good luck with that! Ive done that in the past. Ive a treacle tin with six non inductive resistors in it somewhere, and a BNC on top!
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KI4SDY
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« Reply #35 on: April 27, 2012, 06:19:15 AM »

Until you get your problems resolved with the permanent mount, why don't you try a 3/8-24 stud magnetic mount? You can experiment with both lengths of antennas and maybe keep from burning your transmitter up while you resolve your permanent mount issues.  Shocked

The most likely culprit, looking at your pictures and reading your SWR results, is that the long unshielded center lead is adding electrical length to your antennas. Correct that, and you may be in business. I would replace the rusted antenna mount parts and double check to make sure the shielded leads are grounded properly. I like lug mounted antennas for ease of maintenance and mechanical strength. As far as which antenna will perform best, it depends on whether you are looking for mechanical strength and or the type of terrain you will be operating on. The mechanical simplicity and strength of the quarter wave antenna is hard to beat and this is a concern with your tall van. Grin

If you can't fix it, your going to need new permanent mounts designed specifically for VHF/UHF, if you are determined to go that way. Sometimes, you just have to spend a little more money.  Wink  
« Last Edit: April 28, 2012, 07:40:14 AM by KI4SDY » Logged
W4LGC
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« Reply #36 on: April 27, 2012, 11:16:54 AM »

Having not taken the time to read the many replies preceding mine, I'd just like to note that a centered-loaded 5/8 wave whip is preferred over a 1/4 wave on a vehicle. The reasoning is that the load being elevated up, away from the ground plane provides a better performing antenna system. The low SWR reading could be just confirming that. Use the 5/8 whip, and be thrilled.
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WB6BYU
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« Reply #37 on: May 03, 2012, 02:48:58 PM »

Quote from: W4LGC
... I'd just like to note that a centered-loaded 5/8 wave whip is preferred over a 1/4 wave on a vehicle. The reasoning is that the load being elevated up, away from the ground plane provides a better performing antenna system...


A proper 5/8 whip is NEVER center-loaded, at least on 2m.  That messes up the current
distribution.  The inductor is placed at the feedpoint to cancel the capacitive reactance
due to the antenna being over 1/2 wavelength long.  Anything that shortens the radiating
portion of the antenna (the top 1/2 wavelength of the whip) will degrade performance.

Any 5/8 wave whip that uses a 3/8-24 stud mount is limited in that it doesn't have a
ground connection available.  That means the only form of matching available is a series
coil.  To get a good SWR, this often forces the manufacturer to make the antenna longer
than optimum to get the resistive component below about 100 ohms, even though the
optimum length for low angle radiation often corresponds to resistive components in
the 150 - 200 ohm range (though this also depends on the diameter of the whip.)
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KA1VF
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« Reply #38 on: May 04, 2012, 02:31:29 PM »

I had a situation like this about 30 years ago with a 2 meter 1/4 wave magmount
on roof of a '77 Honda Civic. Good SWR for months until the coax got damaged in
the car door, so I cut off the 4 foot bad section of coax, then soldered new PL-259.

note: apparently the cut off coax was too short and had a weird harmonic length?
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W4LGC
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« Reply #39 on: May 04, 2012, 04:06:58 PM »

WB6BYU,

You are absolutely right. My comment was correct for 70cm, not 2m. Good catch.
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K1CJS
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« Reply #40 on: May 05, 2012, 07:05:28 AM »

If you trim from 1/4 to 1/2 inch at a time, you're likely to go right past the lowest SWR reading without even noticing it.  When you get into the VHF and UHF frequencies, you've got to trim a sixteenth to an eighth inch at a time--and sometimes even less.  On the other hand, if you get the SWR down to 2.5 to 1, most of the time that slightly elevated SWR won't even be noticable to a VHF rig.  The real test is seeing how the rig gets out to a distant station.

I'm not saying you're blowing smoke, but sometimes hams put too much into SWR readings.
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KI4SDY
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« Reply #41 on: May 07, 2012, 07:59:58 AM »

Your antennas were designed to work as manufactured. They may not be a perfect match, but they should be close. I would get your mounts repaired and working properly before I would trim anything. No sense ruining a perfectly good antenna!  Wink
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