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Author Topic: Mobile antenna question - how can I...  (Read 4500 times)

Posts: 66

« on: October 10, 2012, 07:50:34 PM »

Hi all you Antenna Elmers out there--

Antenna design question for you-

How can I build a low-profile high-performance antenna for mounting on a truck roof rack?
Lower than 1/4 wave high from the rooftop...

I have a midsize pickup truck - a Nissan Frontier crew cab with a full size roof racks on it.

I have a 2 meter rig in the truck, and a fold-over MFJ-1422 on the roof.

In the course of my work, I'm in and out of parking decks all the time
and I'm always folding the thing over, or sometimes not bothering and just
not being on the air for days at a time.

I've tried a 1/4 wave (also MFJ) on the roof, and
since I live in the mountains, performance is a good bit less than optimal.
There's a significant difference in how I make repeaters between
a 1/2 wave and a 1/4 wave...

Obviously, the best thing is simply more metal in the air.

The only thing I can think of to do is to find some
low profile but multiple elements design.
I'm happy to drill/mount whatever on the roof rack. Truck has 126k miles, so I'm not looking for something perfect.

I've heard that mounting an antenna on the bed, say in one of the stake-holes or the hitch
would work, but has some downsides when the repeater is through the cab in front of the truck, and half or more
of the antenna is blocked.

So I'd like to find a roof-top solution, if there is one.

Any ideas?

Can I feed four 1/4 wave verticals mounted onto the roof rack?
A loop??


Posts: 2685


« Reply #1 on: October 10, 2012, 08:20:21 PM »

Look for DDRR (Direct driven ring radiator)  antenna designs.  They are less than 4" in height.

Posts: 66

« Reply #2 on: October 10, 2012, 08:56:13 PM »

That's right ---

I remember having looked at one of those for 20 meters back in the day, for mobile use with my TenTec Triton IV

I'll check around.

I'm equally interested in something like four 1/4 wave verticals on the four corners..


Posts: 13010

« Reply #3 on: October 10, 2012, 09:41:46 PM »

Yes, you can make a DDRR less than 4" high, but that doesn't solve the problem
of needing more gain, since a DDRR will be somewhat worse than a 1/4 wave whip.

What are you currently using for a mount?  A permanent NMO through-hole
mount will give an improvement over a mag mount, especially for a 1/4 wave
whip.  (The 1/2 wave antenna has a higher impedance, so less current
through the mount.)

A friend had a similar problem, with only 4" of clearance between the top of his
truck and the garage door.  He put a flexible 1/4 wave whip on top and just let
it bend over every time he went in and out.  The antenna survived for several
years with daily flexing.

Or you could do what I often do - I drive around with a 2- or 3-element quad
for 2m sticking out my sunroof, though that still leaves the problem of retracting
it to go in parking structures.
One possible candidate for such an antenna would be a 2m/440 whip made from
a continuous conductor - the 440 coil then acts like a spring to let the top
section bend a bit more, especially if only the top inch or so hits the roof.

It is possible to use an array of quarter wave whips to make a beam, but
you'll need to have some way to rotate it as you drive.  A 4-way switch
is probably adequate:  look at some of the "4-square" systems for 80m,
and imagine the same approach.  (The phasing and matching networks
would be simpler, however.)  Even mounting 2 antennas along the centerline
of the roof will give some improvement:  for example, mounting them half
a wavelength apart (40" or so) then feeding them in phase gives maximum
radiation broadside to the antennas, and out of phase aims in line between
them (both being bidirectional).  That gives you a couple dB improvement
in both directions, with the disadvantage that you have to switch the
pattern as the road turns.  (Well, sometimes...  the broadside pattern
is considerably wider than the inline pattern.)

Posts: 5917

« Reply #4 on: October 11, 2012, 03:27:05 AM »

Here is a 3.3" antenna:

« Last Edit: October 11, 2012, 03:42:57 AM by WX7G » Logged

Posts: 12667

« Reply #5 on: October 11, 2012, 05:06:26 AM »

"There's a significant difference in how I make repeaters between
a 1/2 wave and a 1/4 wave..."

It looks like you have already defined the minimum acceptible. I doubt that you'll find anything that is lower profile than a 1/4 wave whip and yet has the gain of a 1/2 wave whip. I'd try some larger antennas mounted on a rear corner stake pocket. It'll be more directional than an ideal center-roof installation, but it may be an acceptible compromise given the need for low profile.

The ideal antenna is not of much use if its always removed so you can get in and out of a garage. A lesser antenna may win out if its always available when you need it.


Posts: 124

« Reply #6 on: October 11, 2012, 05:57:45 AM »

Not the cheapest option, but you can find them in the classifieds here, QTH, QRZ, or hamfests from time to time.

Don't even have to get out, you only have to remember to hit the switch.

Posts: 5917

« Reply #7 on: October 11, 2012, 06:25:43 AM »

The difference in gain between a 1/4 wavelength monopole and the MFJ-1422 is 3 dB. You might go with the 1/4 wavelength and increase your power by 3 dB.
« Last Edit: October 11, 2012, 06:27:59 AM by WX7G » Logged

Posts: 66

« Reply #8 on: October 11, 2012, 06:27:16 AM »


Isn't it possible to make 1/2 and full wave DDRRs?

I have heard that the DDRR is less efficient compared to an antenna of it's same wavelength - and it makes sense too.

I'm wondering if I were to make the thing longer than 1/4 wave, it'd perform better.
I think it's certain that a 1/2 wave 146Mhz DDRR won't perform as well as the MFJ 1422 at 1/2 wave all by itself.

But if it will improve over a 1/4 wave vertical, I might have something. Depending upon just how ferociously ugly and wind-resistant the thing is!
While I'm an engineer and not overly given to vanity and how my truck looks, I suspect there _are_ some limits on just how far I want to go with
weird looking assemblies strapped on the roof rack  ;-)

I probably should have mentioned that with the 1/4 wave, I actually don't even make a couple of my commonly used repeaters
on my drive to and from work and other locations.
A little noise and less than full-quieting I could handle.
Not even being intelligible and having people ask me to repeat things isn't all that nice for them.

I don't mind a bit of a whang when entering a garage - it happens with the 1/4 wave, and it's just the last inch or so.
It'll survive for years that way.
But, I've seen garage attendants come out and check overhead lights and hanging signs for damage after I've gone by, so I don't want to be blamed for some damage I might not even have done, someday...

Posts: 5917

« Reply #9 on: October 11, 2012, 06:32:27 AM »

The DDRR is an inverted-L formed into a loop. It is the short vertical section that radiates and if done correctly will perform as well as a 1/4 wavelength monopole.

A half wavelength monopole has gain over a quarter wavelength monopole because the magnetic field from different parts of the antenna combine such that the radiation pattern is compressed. Bending it over (such as a longer DDRR) defeats this.

So, how can an MFJ-1422 be mounted and not hit objects? How about one mounted on a front fender and one mounted in stake mount? Using a two position RF switch, switch antennas as needed.

« Last Edit: October 11, 2012, 06:35:33 AM by WX7G » Logged

Posts: 66

« Reply #10 on: October 11, 2012, 06:33:15 AM »

The difference in gain between a 1/4 and a 1/2 wavelength monopole is 2 dB. You might go with the 1/4 wavelength and increase you power by 2 dB.

My immediate impression from the mountain area I live in is that the 2dB is being added to/subtracted from by terrain
by quite a bit.

Depending upon where I am, the 1/4 wave either performs so well that I and other parties on the QSO can't even tell the difference, or
I've gone down into a valley far enough that the 1/4 wave really doesn't cut it.

Those valleys, of course, aren't great performance even for the 1/2 wave MFJ, but if propagation and other factors (repeater site intermod)
are normal, I can still be heard clearly enough with the 1/2 wave, at the full output of my radio, which is 60 watts.
If intermod is up to abnormal levels at the repeater site, there are a lot of folks who don't make it in from the valleys, and that's just life here.

The lab-conditions effect of the  2dB difference doesn't seem to be completely linear out in the field, which is  I guess, expected, given that it's not "the lab" Cheesy

Posts: 66

« Reply #11 on: October 11, 2012, 06:35:31 AM »

All being considered, I guess I should probably just try mounting the MFJ in a stake-hold and seeing how that actually does.

I am operating on an urban legend of sorts that the cab of the truck will seriously interfere if it's between the repeater and the antenna...

Posts: 237

« Reply #12 on: October 11, 2012, 07:12:54 AM »

Hi OM Tim

I guess something like this would serve your purpose.

It would have to be mounted with the blade broadside to the travel direction, of course!



Posts: 5917

« Reply #13 on: October 11, 2012, 08:38:10 AM »

The army antenna looks like it would blow down with the vehicle in motion.

Two MFJ-1422 antennas with one on the front fender and one on a bed stake mount switched with a coax switch might meet your requirements.

Posts: 1631

« Reply #14 on: October 11, 2012, 08:45:52 AM »

I built two identical DDRR"s and placed the second one a quarter wave apart and we measured gain expressed in Field strength in the direction of the second DDRR.

The gain was within reasonable expectations, and noticeable during on air rapid switching.

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.

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