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Author Topic: Mobile 2 Meter 1/4 Wave Whips Have High SWR but 5/8 Checks Fine?  (Read 9411 times)
G7MRV
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« Reply #15 on: April 19, 2012, 09:24:49 AM »

Do you have a test load (dummy load)? If so, firstly connect it directly to the analyser and make sure that it reads 1.1:1 or there abouts, and shows a flat response. This will rule out the analyser as a source of error.

If that checks out ok, then try connect the test load to the antenna mount. This might prove tricky as you have a 3/8 mount, but if you can find a way to do so that still looks electically 'coaxial' (ie the ground connection is very short) then the impedence discontinuity shouldnt be too bad. If you can do this, and get a 1.1:1 or thereabouts reading, then you know its the whips themselves. I admit this might prove impossible to do.

I have a personal feeling that your going to find the analyser is reading high. This might be why the 5/8th reads 1.7:1, when it might very well have been trimmed to present 1:1. A 0.7 high error would mean your lowest reading on the whip of 2.3:1 would in reality be 1.6:1! Almost exactly where you want it!. Are you able to borrow another analyser, or have you a normal VHF SWR bridge you can try? 2:1 isnt going to harm your radio on low power for the few seconds it would take to check with an SWR meter.

A big problem we have as amateurs is that we have to rely on our test equipment to be accurate, when in reality amateur equipment will rarely be within 10%, and is unlikely to have been calibrated except in the factory. I have a marconi 2955 test set which i occasionally take and calibrate against known accurate equipment at work.
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AC4RD
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« Reply #16 on: April 19, 2012, 09:27:28 AM »

one whip, it was resonant (at least 2:1 or something and X=0) around 120 MHz. I slowly trimmed until the "resonant" spot worked it's way up to 146 MHz but the lowest SWR at the point even was 2.3:1

JST, I'm just guessing and I'm no expert--and I don't use VHF at all, either.  But it's pretty routine for my mobile HF antennas to get to a nice resonant point where I want them, and then the measured SWR isn't great and I need a bit of matching at the feedpoint--capacitor or a small shunt coil.  Could it be that you're resonant but need some matching to get the SWR down?
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G7MRV
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« Reply #17 on: April 19, 2012, 09:33:38 AM »

one whip, it was resonant (at least 2:1 or something and X=0) around 120 MHz. I slowly trimmed until the "resonant" spot worked it's way up to 146 MHz but the lowest SWR at the point even was 2.3:1

JST, I'm just guessing and I'm no expert--and I don't use VHF at all, either.  But it's pretty routine for my mobile HF antennas to get to a nice resonant point where I want them, and then the measured SWR isn't great and I need a bit of matching at the feedpoint--capacitor or a small shunt coil.  Could it be that you're resonant but need some matching to get the SWR down?

A 1/4w whip on a decent groundplane should present about 30 ohm resistive and 0 ohm reactive at resonance, equating to a VSWR of 1.5:1. There is no call for any further matching at this point, as there is no reactive componant to match out. If you add matching to get a 1:1 VSWR then you are actually increasing the reactance.
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G7MRV
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« Reply #18 on: April 19, 2012, 09:43:30 AM »

Just been rereading the thread, picked up on a couple of points

Recapping -

You have tried 3x 1/4w whips, none will go below 2:1
You have tried these on two different vehicles, hence two different mounts
You have used two different analysers, a WaveNode and a MFJ-269
You have a 5/8th antenna that works and checks out fine on both mounts

What we need to look for is the common factor that we are overlooking here, and its probably something simple. Now, I dont mean to be rude here, but one common factor is the operator! You have been trying for a while, so you may be overlooking something.

What other common factors are there? Are you using an adaptor in the RF connection? Are you powering the analysers off the same PSU? Look for anything that is common that might be influencing the results. Even go so far as to rule out the tape you used to measure the whips! I have a number of rulers/tapes which do not start at 0! I also have a tape from Rhode and Schwartz marked in cm and frequency!
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WB6BYU
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« Reply #19 on: April 19, 2012, 02:48:59 PM »

How long is the cable between your SWR analyzer and the mount?

You can't count on X = 0 meaning the antenna is resonant unless the coax is of ZERO length,
or you correct for the actual coax length.

Instead, look for the frequency of minimum SWR.  That is a much more accurate method of
getting minimum SWR for the rig than tuning for X = 0 through a random length of coax.
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W4JST
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« Reply #20 on: April 21, 2012, 05:32:04 PM »

G7MRV, AC4RD, WB6BYU and everyone else who responded: Thank you!
I appreciate the helpful responses.
I think there may have been a problem with how I had my mount installed. Maybe someone can tell me what was going on:

I have two mounts on my van, one in the center for the CB antenna, and the one near the rear where I have been testing 2 meter antennas.

I decided I wanted the 2 meter antenna to be in the middle of the roof, hoping it may increase performance, so I removed the CB antenna and cut the coax and rerouted the appropriate length to the 2 meter radio location.

Prior to cutting the coax or making any changes, I checked the 1/4 wave and 5/8 wave 2 meter antennas in the center position, where I previously used my CB antenna, and they checked the same. The 5/8 whip was okay but not the 1/4 wave.

But looking at how I installed the coax to the mount, I thought a couple of improvements could be made.

Go to: http://www.myalbum.com/Album=7OADAA37
The first picture is the rear antenna mount I haven't changed yet.
Two things I didn't like, the center conductor was unshielded and inside the vehicle for a few inches. I thought this might radiate inside the vehicle more and, being outside of the shield, begin to act as part of the antenna length. I don't know. Maybe someone can guide me in this department.
The second thing I didn't like, especially on the center mount, where I drilled the screw for the shield/ground connection of the coax, was on a piece of metal inside the van, again thinking it might radiate more inside the van, I don't know, but also, it wasn't directly attached to the roof of the van. I thought maybe a connection should be made more direct, so the roof directly under the whip acts as it's ground plane, rather than some metal beam inside the van somewhere. I don't know if this was the problem or not.

In the second picture, you can see the cross piece I was referring to, and the hole where I used to have the shield of the coaxial cable attached. The second picture shows how I did the mount now, with less center conductor exposed out of the shield, and I made the shield/ground connection, directly at the roof right below the antenna.

Once I did this, my 5/8 whip antenna checked short and seems resonant up around 151 MHz or something, maybe I just need to adjust the whip, I don't know.

I then checked my three 1/4 wave whips and they made more sense. The first two being two short, and the second one that I have never trimmed is just a little long with SWR being around 1.7:1 at 146 MHz.

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WB6BYU
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« Reply #21 on: April 21, 2012, 08:52:29 PM »

Quote from: G7MRV

... I use an MFJ-259B, and i watch for the X reading to be as close to 0 as possible, indicating very low reactance, and hence resonance, rather than watching the SWR reading.



But if you have a random length of coax, the X = 0 reading doesn't necessarily tell you anything
about the resonant frequency.  If the antenna is 30 + j0 ohms (resonant) and you measure it
through a 3m length of RG-58 coax the impedance comes out to 68 + j18 ohms, which would
appear to be non-resonant.  By the time you prune it for X = 0 at the analyzer you're actual
resonant point is outside the band.  Even 10cm of coax would transform the impedance to
34 + j14.  You'll only get X = 0 at the analyzer when antenna is resonant if the coax is a multiple
of an electrical quarter wavelength.

That's why SWR is a better indicator when measuring through a random length of coax.



Quote from: W4JST
...
Two things I didn't like, the center conductor was unshielded and inside the vehicle for a few inches. I thought this might radiate inside the vehicle more and, being outside of the shield, begin to act as part of the antenna length.



Both could be a problem, more so at VHF than HF.

The radiation inside the vehicle isn't really a problem, but the length of the BOTH the center
conductor AND the shield contribute to the electrical length of the antenna.  The inductance
of the ground lead also raises the RF voltage on the coax shield above ground, so you'll
have more common mode current.  (This will contribute more to radiation inside the vehicle
than that from the actual wire length itself.)

The ground connection should be to the metal roof right at the antenna.  This isn't easy for
some of the common 3/8-24 mounts, which is why they aren't used as much on 2m and higher
frequencies.  The second one appears to use a lug right against the metal around the roof,
and is a much better approach (as long as you have a toothed lockwasher to maintain good
electrical contact through the paint.)  The ground wire to a separate piece of metal, which
may or may not be thoroughly grounded to the roof, isn't a good idea at VHF because the
route for the RF between the coax and the metal roof may be quite long in terms of wavelength.



Also, the bolt looks rusty.  I hope you've cleaned the contacts and applied some anti-corrosion
grease to it.
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W4JST
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« Reply #22 on: April 22, 2012, 10:08:53 AM »

Thanks for the information. I think basically changing that ground to directly under the antenna rather than on that beam running under the roof made the difference.

The rear one is rusty. While they don't leak, I guess especially without an antenna in them and maybe even with an antenna in them, those bolts will rust.

While I had the middle, now 2 meter position apart, I put a new mount in. It's not rusty, at least for now.

I have read in the past that the best place, to protect the passengers from radiation, is to mount your antenna in the center of the vehicle's metal roof. I understand the car body/metal is radiating itself, so is it shielding the passengers from the radiation or not? Does what I said previously about the length of the center conductor prior to the antenna mount, or running the ground to somewhere else inside the vehicle, rather than directly under the antenna, have any effect on radiation inside the vehicle?

Is there any information I can find regarding radiation safety etc?
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G7MRV
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« Reply #23 on: April 22, 2012, 01:12:14 PM »

The wide gap and the long tails in the first one would have introduced a large impedence discontinuity, which would have made an accurate match almost impossible. Using a connection designed to be coaxial would be best, but reducing the length of the tails and the distance between the contacts would serve as well, as you found with the other mount. It is also essential to ensure that the ground connection point and the 'groundplane' are one and the same, ie the ground connection should be to the metal through which the mounting is fitted, any other place could well either not be bonded in at all, or have some other discontinuity.
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W4JST
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« Reply #24 on: April 22, 2012, 05:09:20 PM »

I think you're exactly right. I have installed a lot of these mounts for people's CB antennas etc. and was not aware of any problems. It seemed to make a difference this time at least on my 2 meter antennas. I did not even know it was my mount installation.

Perhaps it was just the length of wire out of the coax to make the connections. I believe the cross member which I ran my ground wire was probably spot welded to the rest of the van body, but did not directly attach (would have to take a longer path) to connect to the metal roof directly under the antenna.

Thanks everyone.
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G7MRV
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« Reply #25 on: April 23, 2012, 12:12:39 AM »

Glad to see you've got it sorted. Incidentally, I did almost the same antenna work over the weekend - cut a 1/4w to use occasionally in place of my 2m 5/8th, but i use a professional public safety antenna mounting, which is completely sealed and coaxial.
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W4JST
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« Reply #26 on: April 23, 2012, 06:22:05 AM »

How do the two (1/4 wave versus 5/8 wave) compare?

Some people have told me it will be a big difference using a 5/8 wave but from what I've read it may not even be noticeable.

Now that I have a second antenna mount installed for my amateur radios, I may use a 5/8 in one spot and a 1/4 in another and have an antenna switch for comparison purposes. Will the location on the roof make much difference?

Also, with my little 1/4 wave in the middle of the roof, will have a longer antenna in front of and/or behind it, hurt performance any working like a reflector or anything?
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G7MRV
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« Reply #27 on: April 23, 2012, 09:21:16 AM »

My VHF antenna mount is dead center of the roof. I made up the 1/4w to have something that would be serviceable if required on 70cm. From my location i can access a repeater some 30 miles away next to my workplace, from my driveway outside the house. With the 5/8th the repeater is S9+20, on the 1/4w it was S9. So there were a couple of S-points difference. This is hardly a quantative test (which i could do if i get a mate with a calibrated Tx to give me a hand) but shows that except for reasonably weak signals theres not a lot in it. The 5/8ths has a much greater aperture (capture area) but that also means its much taller and more noticable. Im sure my wife prefers the 1/4w! The 5/8ths also cant be pressed into use on 70cm.

As for the effects of other antennas, yes they may have some effect on the pattern, but i doubt it would be of much concern, however if both antennas are cut for the same band the one out of service may act as a parasitic element. Experiment is the best action here i think.

I dont currently have another antenna on my vehicle, but in the next few days another mount is going on, near the rear. This will be used for HF antennas, and as these are 3m long, they make the 5/8ths look small!
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WB6BYU
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« Reply #28 on: April 23, 2012, 11:39:58 AM »

Quote from: W4JST

How do the two (1/4 wave versus 5/8 wave) compare?

Some people have told me it will be a big difference using a 5/8 wave but from what I've read it may not even be noticeable.


In my experience about 1dB difference, measured by switching between the two
antennas while driving through difficult conditions.  Basically, if the signal isn't noisy on
the 5/8 wave, it will still be copyable on the 1/4 wave.  In a crunch it makes the
difference between, say, 40% copy and 60% copy:  the 5/8 wave is a bit easier to pull
through in marginal conditions, but even that advantage may be offset by the
polarization shift as the whip bends at high speeds.



Quote

Now that I have a second antenna mount installed for my amateur radios, I may use a 5/8 in one spot and a 1/4 in another and have an antenna switch for comparison purposes. Will the location on the roof make much difference?



A little bit.  Not so much with the 5/8 wave, because the roof doesn't extend far
enough in any direction to simulate the infinite groundplane over which the 5/8 wave
whip gain myth was developed. 

What makes more difference than the location on the roof, or even the choice of antenna,
is exactly where you are parked.  I can measure either antenna as being several dB better
than the other with both antennas mounted on my roof, then move a few feet and see the
same advantage with the other antenna.  This is because any multi-path interference will
affect the two antennas differently even with an offset of less than 1/4 wave.  This is
why I make my comparative measurements while driving through marginal coverage area
and switching back and forth between the two antennas - I can get a sense that one tends
to be a bit better or a bit worse, but the signal strength bounces around a lot with both.

If you are trying to take a precise measurement you'd want to compare the two antennas
on the same mount in a fixed location.  Using a step attenuator I can usually judge the
difference within about 1dB.  (I have equipment that will detect a change of 0.2dB, but
it has never been worthwhile trying to compare the antennas that closely.)

Note also that a true 5/8 wave antenna will have an impedance between 100 and 200
ohms.  Some manufacturers make them a bit long to give a better SWR, which raises
the radiation pattern.  So you may notice a difference among different models.



Quote from: 7MRV

...With the 5/8th the repeater is S9+20, on the 1/4w it was S9. So there were a couple of S-points difference...



You have to be very careful using the S-meter to make comparisons, particularly with
rigs designed for VHF FM.  They often have just 7 actual segments on the display (even
though it may be marked to make it look like more) and the ones that I've measured only
have about a 10 to 12dB range from zero to full scale.  One of mine has markings at
S9, S9 + 20 and S9 + 40, but the last two are on the same step so they come on
together.  This means there is only a 1dB to 2dB difference between S9 and S9+40 on
that rig.

Using a step attenuator to check the relative levels where the steps change is a good
starting point - that will give you a better idea of the differences.



Quote from: W4JST

Also, with my little 1/4 wave in the middle of the roof, will have a longer antenna in front of and/or behind it, hurt performance any working like a reflector or anything?



If it does, it won't be enough to notice.
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W4JST
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« Reply #29 on: April 23, 2012, 11:54:53 AM »

G7MRV and WB6BYU thanks a lot!

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

I think I may take an antenna switch and switch back and forth between the two antennas for comparison purposes.
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