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Author Topic: Mobile antenna question - how can I...  (Read 5070 times)
W5WSS
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Posts: 1744




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« Reply #15 on: October 11, 2012, 08:52:35 AM »

Incidentally the pattern developement can be with two loaded verticals to fit your garage
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KA4LFP
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Posts: 66




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« Reply #16 on: October 11, 2012, 12:14:21 PM »

Errrmm... Huh??

I'm not at the level of antenna work to know how to run complex design and modelling software and so on..

Are you saying that I could simply mount two 1/4 wave verticals front and rear of the roof, and use a T connector in between them, and it'd appear to the transmitter as the correct impedance and resonance?

I can see how there'd be a pattern (evidently forwards and backwards?) resulting from two verticals like that..
Which might be OK..

Just some food for thought: One could add a vertical quarter wave and turn the major lobe using the vehicle steering wheel if installation prohibits controlling the pattern.

Use a UHF coaxial T connector for interconnection between the two verticals
Find the lobe point and shoot.

Model a pair of co/phased verticals for broadside gain then decide if the idea fits the criteria. good luck and have fun.

73
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WB6BYU
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« Reply #17 on: October 11, 2012, 01:30:04 PM »

Not quite that simple, but perhaps not too much more complicated.

Two quarter wave verticals spaced fore and aft 1/2 wavelength apart can be matched
using some 1/4 wave lengths of 75 ohm coax. 

The pattern is bidirectional broadside to the antennas (off the sides of the car) when
the antennas are in phase, and in line with the antennas (front and back of the car)
when they are out of phase.  Making them out of phase would involve switching a
1/2 wavelength of coax into one side, or a 1/4 wave piece that is added to one side
or the other with a switch.  This part isn't difficult to design or build.

Because the pattern is directional, it has perhaps 2 to 4 dB gain in the favored direction,
and nulls in other directions.  That means you have to switch between the patterns and
choose the one that gives the best signal when you change directions.  (And possibly
other times, because you're not always picking up a direct path in mountainous terrain.)


I drive though marginal signal areas in the hills every day on my commute, which gives
me an opportunity to compare two antennas by switching between them as I drive.
In spite if manufacturing claims, I haven't yet found any reasonable length mobile whip
that averages more than about 1dB better than a quarter wave ground plane when mounted
on my van roof.  Such comparisons aren't as easy as they might seem:  in the presence of
reflections the exact position of the antenna makes more difference than the antenna
length itself.  In one case I measured a 3dB difference, then moved the van forward a
foot and the antenna that had been better was now 3dB worse.  So I have a switch
and flip back and forth between the two antennas, checking to see which one tends to
give better signals.  I certainly haven't found any antenna up to 4' tall that enables
me to hit repeaters that I can't hit with a quarter wave:  usually the difference is
between 40% copy and 60% copy, or similar.  That leads me to think that the quarter
wave whip you tested might not be up to snuff.
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W5WSS
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Posts: 1744




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« Reply #18 on: October 11, 2012, 04:54:06 PM »

Yeah to wb6byu. I have done similar evaluations with two verticals from a hill top where I could turn the vehicle on a radius(rotate) and aim the broadside pattern towards the target station. Then compare the differences with a single quarter wave removed. completely.

The reporting station was non skywave mode for better accuracy.

Found plenty of signal increases via rapid switching while stationary at the 50 mile range to suggest what has been known for many years to be a worthwhile effort adding a second vertical upper hf to vhf is feasible.

The higher frequencies physically work for optimal proper spacing fore and aft. on an automobile.

I dialed it in and resulted in a better overall system with the two verticals fit on the auto. than one.
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W8JI
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WWW

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« Reply #19 on: October 11, 2012, 07:47:43 PM »

Let's get the facts straight about the DDRR.

The DDRR only radiates from the short vertical section.  The entire horizontal loop area of the antenna is nothing more or less than a simple top loading stub.

If you build a 4" tall DDRR, you have a 4" tall top loaded vertical. Period.  That is all. The radiation resistance of the DDRR is exactly the same as any top loaded antenna the height of the ring from the groundplane.

If the DDRR is built from large low loss conductors, because it is top loaded, it works OK. It can get close, with a low loss groundplane and low loss conductors, to a 1/4 wave monopole exactly like any other top loaded vertical could do.

The poster says he has a problem with a full 1/4 wave antenna, and the DDRR will have less field than a 1/4 wave no matter how we try to spin the numbers.

His problems also might be ground clutter clearance as much as gain. Many taller antennas have far better return than the gain indicates because the radiating area is elevated more. Just 20 more inches height from earth surface to the current maximum point can be significant when the height above ground is only 40 inches to start with. It often means far more than the 1dB to just under 3 dB gain a taller radiator provides.

Anyone seeing a large difference between a 1/4 wave and a half wave, providing something is not wrong with the 1/4 wave to cause it to seriously under-perform, almost certainly has more than a gain issue going on.

A magical fantasy antenna might help with overhead obstruction clearance, but it will have less gain and effective radiation center height than a 1/4 wave and certainly not act like a half wave. The ONLY radiating area of the DDRR is the short vertical conductor.

73 Tom

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N6AJR
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Posts: 9910




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« Reply #20 on: October 13, 2012, 12:13:29 PM »

Perhaps not the best antenna in the world, but it may do what you need. an ATAS 120 screwdriver antenna, and an electric tilt over mount ( I think diamond makes a pl 259 base tilt over mount) and use it to lower the antenna when needed and  put it up when driving. A fishing line  "guy" for when its up will help take some strain off the mount.
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W5LZ
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Posts: 477




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« Reply #21 on: October 13, 2012, 10:48:50 PM »

Mount a 5/8 wave on the front fender and quit worrying about it.  Shouldn't be a problem with parking garages, and should work as well as a typical 1/4 wave in most cases.
 - Paul
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W8JI
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Posts: 9296


WWW

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« Reply #22 on: October 14, 2012, 04:19:06 AM »

The 5/8th wave or 1/2 wave verticals obtain their gain because the current area in the antenna is elevated higher above the groundplance. Without a flat groundplane extending out some distance a 5/8th wave has no gain, it can even have loss.

Put a 5/8th wave low on a vehicle so only the upper part clears the sheet metal, and pretty much every advantage goes away. You might as well just use a 1/4 wave on the roof as a 5/8th on a fender.
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KA4LFP
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Posts: 66




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« Reply #23 on: October 14, 2012, 07:16:17 PM »

The 5/8th wave or 1/2 wave verticals obtain their gain because the current area in the antenna is elevated higher above the groundplance. Without a flat groundplane extending out some distance a 5/8th wave has no gain, it can even have loss.

Put a 5/8th wave low on a vehicle so only the upper part clears the sheet metal, and pretty much every advantage goes away. You might as well just use a 1/4 wave on the roof as a 5/8th on a fender.

Thanks for your thoughts, everyone-

I think I may have to check into the performance of my 1/4 wave some more, and consider the dual-1/4 wave, even if it's not truly omnidirectional.

W8JI has given me the background for what I'd thought true all along -- that fender mounting my 5/8 wave would have some drawbacks - I see two of them mentioned here:

- off-center groundplane, which I don't have right now with the center of a 4' x 5' metal roof as a groundplane. I might have a problem with groundplane once I moved to the extreme side of the vehicle right against the fender.

- radiation point would definitely be at the same height as the 1/4 wave, or maybe even slightly lower.
Paul is correct that it may work as well as the 1/4 wave at the roof height for that reason.

So that solution would solve my height problem, but would possibly leave me with the reduced repeater quality-of-signal that I get with the 1/4 wave.
Should not be hard to try and find out, though.

Turns out I don't have stake holes in the truck! I thought there were some down underneath the bedliner, but there aren't in the Frontier.
I can mount something back there though, as a test.
Just makes life challenging when I'm trying to carry a load of lumber or plywood - one more thing to worry about the load damaging..
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