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75m-80m MOBILE ANTENNA - WHAT HAVE YOU MADE?

C

CDX-007

Transducer Fanatic
Jan 26, 2006
706
17
28
I like running 75m / 80m when I'm mobile (400mi / wk) but it's darn tough to get any performance from an antenna which has to clear 13' 6" overpasses on a band where the 1/4 wave is 60 feet.

I have screwed with mag mount Hamsticks centered on the roof and they 'kinda' work. Used a Don Johnson DK-3 and it worked a lot better.
I have a friend with an High-Q 6M-160M monster with a paper towel-sized load and it is a little better than the DK-3 on low band but I think I want to build one with an even larger coil and a 66" W-1000 whip & 3' cap hat, unless there's another design out there which will outperform it.

BTW, I have a 500w HF mobile amp.

My idea is to wrap 3/8" - 1/2" C0PPER or ALIMINUM tubing into a coil about 18" diameter & as many turns as it takes to tune to 3.6mHz and having a feed wire w/alligator clip to tap the coil at the appropriate location for tuning 50ohms up to 6m.

I expect I can find a location on the coil where it will tune 1/4 wave, 1/2 wave, 5/8 wave, or 3/4 wave depending how high I go in frequency, and I am planning to ground the bottom of the coil.

Has anyone played with this design or a similar one, and if so, what have you built & how well did it perform?

Thanks & 73
 

Sounds a lot like a 'BugCatcher'. The only thing that needs to be heavier than 'normal' would be the bottom mast under the loading coil. The coil certainly doesn't need to be made from 3/8" or 1/2" copper! Ordinary #14 or maybe #12 wire should work just fine. Make the spacing between turns of the coil slightly larger than 'normal' to handle that 500 watts (maybe 4 turns per inch?).
A capacitive hat needs to be a ways above that loading coil, a foot or two, more is better sort of. I'd also think about using an impedance matching coil at the base of the mast. If this will be a single band antenna, find the right tap point and solder it. That matching coil will work out much easier than a torroidal one. At about 500 watts, a torroidal matching coil is gonna be sort of big, otherwise it'll saturate.
Tapping the loading coil is sort of tricky too running that kind of power. A small alligator clip tends to not last very long, corrosion mainly.
Lots of different ways of doing things, use the ones you're most comfortable with...
- 'Doc
 
I've seen a few mobile setups like the Tarheel/Screwdriver type antennas using a trailer hitch mount. There's even one you can use that will still allow you to tow a trailer.

Personally, I think the trailer hitch mount is the best compromise to really get the signal out, without interfering with the overpasses/trees. Also, this way, you have a direct ground to the frame, so interference is minimized.
 
In all honesty, if you aren't getting performance out of a Hi Q,
THERE IS SOMETHING WRONG! Of course, size does matter..................but! You CAN get past a point of diminishing returns! To be sure, it is hard to "scrunch" 100-200 feet of wire down into a 9' package, and the fact of the matter is, efficiency suffers. However, I think Doc is on the money.

You are trying to go WAY overboard with your monster "10K clone" and it is likely not to work. Here are a few thoughts: use 'em or discard 'em.

The optimum USABLE HF loading coil is around 3-4" diameter. Yes, size would seem to be the way to go. However, as the "Q" of the coil increases and the bandwitch narrows (as it WILL as your coil gets bigger), it will also begin to become more and more sensitive to the surroundings thru which it has to travel :( This may mean that your coil will severely de-tune and do it to the point that it becomes unusable as it encounters rapidly changing environments. Your better "average" usable coil is similar to Henry Allen's #66 Bugcatcher using #12 wire at 6 TPI. Set up properly (and I, too, prefer the coil at the feedpoint) this setup will ROCK! I PROMISE it will. A 3" screwdriver is BETTER with the single most important advantage being that the screwdriver can adapt to the changing environment because it can fluidly tune to MEET said changes while the fixed-tuning loading coil cannot! I have build PLENTY of the bugcatchers and plenty of the 2 and 3 inch screwdrivers, and I KNOW that they can be made to really perform well without being HUGELY sized. True, they might have to be 9 feet long, but there's ways to get around that as well. One way for a big truck is, if you have a fiberglass cab, put the darned thing INSIDE the cab on the back wall, allowing one end to be near the whip mount on the cab body! :shock: Yes! It CAN be done!!!!!! Allow the screwdriver body to attach horizontally to the back of the cab, run a short wire over to the whip and let the whole thing "count" as whip. I have some pictures of a fellow's camper where he did this and it's pretty neat. The only thing showing is what looks like a CB whip mounted directly to the fiberglass camper shell! :shock:

Something to think about!

73

CWM
 
OK! Some excellent info. guys, thanks!!

I guess I'm going to have to build several and test for best efficiency. Oh boy.

My thought was to immitate the Wilson 1000's performance on a band 7 times longer in frequency. I was toying with the idea of 15 turns @ 15" / turn + 77" or so, too bad I can't get 7 times the 66" whip!
Maybe if parked...

Also, I was hoping to fit it on top of the 62" high roof for a slightly better counterpoise, especially on the higher bands.

Another idea was to build two more like the DK-3 and parallel them, one per side of the Outback. I have heard this would provide considerably more than 3dB due to two antennas approaching, but less than, a 1/4 wave radiator. Anyone try phasing two for 80m?

And, ...what do you think I'd see with a 15 turn coil, 3 turns every 2", & 15" in diameter?
 
Personally, I think you'd see a mechanical nightmare. A coil that size just isn't very practical or needed. Something like a 6" diameter coil, maybe 4 to 6 turns per inch, is the 'practical' limit before those 'diminishing returns' really start taking a bite out of things. Sure, you could make a coil 3 feet in diameter, but what would be the point? It's efficiency just isn't gonna be that much greater than that 6 - 8 inch coil.
Efficiency! The average efficiency of a very well designed 80 meter mobile antenna is something on the order of maybe 1%... fudge a bit and call it 3% (if you are very, very lucky). That deals with 'radiation resistance', which is an imaginary way of looking at it, and while it's not exactly something you can take to the bank, it does make things sort of simpler, sort of :). Also deals with how much an antenna is shortened in relation to wave length. Just take my word for it, that 1 - 3% efficiency on 80 meters is certainly 'ballpark'. That 1 - 3% efficiency is actually enough to do just fine in most cases. (That's from over 20 years of doing 80 meter mobile, by the way. WAS several times, a Canadian WAP (provinces), I think a WAC, and all on the bottom end of 80 meters...sort of. No, it isn't bragging, there are/were quite a few others who did the same thing. (Geratol'ers)
- 'Doc
 
For mobile HF operation, the greatest source of loss on the lower HF bands is from inadequate ground plane, or "image plane" or "counterpoise" if you prefer. The vehicle's performance as an image plane for the antenna is improved by braided-strapping chassis panels together, and mounting the antenna on the highest point on the vehicle, or at lease the coil must clear the roof.

Also the vehicle chassis capacitively couples to the earth below it, and capacitive reactance increases on (decreasing frequency) longer wavelength bands.

The RF ground plane is half of your antenna. It's the first priority.
The next most important is the coil you were discussing.

Anyways, i've got a wilson 5000 on my car for 10/11 meters. I'd love to have an 80 meter antenna that i could swap in its place that would screw to the same mount.
 
OK, I hear you.

- But, since I want an antenna which will radiate as much of my 500w or so as possible I think I'm going to have some fun with a 12" diameter coil @ 20 turns, 6 guage C0PPER, 5 turns every 2" which will leave approximately a 5/16" gap & an 8" tall coil.

It will be huge, ugly, and scream "LOOK AT ME, I'M A RADIO GEEK!", but it should perform well, I hope, and either way it will be fun to do something a little radical.

66" + 18" to the bottom of the coil + 4" of coil + 72" whip = 160" or 13' 4". - I'll post pictures.

I'm wondering if having only an 22" high coil with a 72" whip & cap hat might be handled by a 4-magnet mount?

Also, I think I'll buy 3 more Wilson whips and cut off the top ball so I can bend them around into a 5.5' perimeter, 3-leaf clover capacity hat.

This will be mounted to the center of the 66" high roof and grounded to the chassis. The bottom of the car drags 6 C0PPER ground straps which help bleed off static, and I was even toying with dragging a 60' black counterpoise wire, but this is California so I guess I better reconsider or they might arrest me for being some kind of public wire-dragging terrorist. :roll:

I am going to take your advice and run grounding straps to all the fenders and doors, and maybe even wrap & ground the dang loud electric fuel pump.

I'll be drilling and 'tapping' the bottom of a solid 1" fiberglass rod for the base and then I'll epoxy into the base a 3/8 x 24 stud & several well-tightened hose clamps around the base for added torque strength where I introduce the stud.

I hope to QSO all around the country so maybe I'll hear you on the band and thanks guys for all the helpful info.

73
 
Even with 20 turns of 6 ga. 12 inches in diameter I don't think it will be self resonant on 80m.Mobile antennas on 80m need a lot more turns than you think. Do you have an antenna analyzer? It will save a lot of headaches if you do.

I did make an antenna for mobile use and run it with my FC-40 auto tuner. It is self resonant on 15m. It consists of a 48 inch fiberglass bottom section 1/2 inch at the bottom and 3/8 inch at the top. It was a quad spreader from a Wilson Shooting Star. It is wound with the tinned braid from RG-8 coax at approx. 1 turn every 3/4 inch. Everything is covered in one continous length of black heat shrink tubing.I finished up the ends to accept standard 3/8-24 male threads on the bottom and female on the top.I used 3/8 antenna mounts that I modified a bit. Next I used a 6 inch fiberglass insulator with 3/8 threads on the bottom and a whip adapter on top. It came from an old Radio Shack CB antenna.I wound it with the braid similar to the bottom section.I used the braid so as to reduce losses as much as possible. Wire would work but would have higher losses due to the smaller size. On the very top I use a 58 inch whip from the old CB antenna.The whole thing is 9 foot 4 inches high and is mounted about four feet off the ground on my Ford Escape.It works GREAT on 20m and up,not too bad on 40m but summer time use on 80m has not been too good. I will wait and see what the winter brings. I used the 6 inch insulator BTW so I could change it out with various coil arrangments and see what works better. My experimental time however has been somewhat limited lately so I have just left it as is.The one thing is that the bottom section is VERY stiff while the top whip lays back a lot at highway speeds but the flexibility helps out when it hits the tree branches in town. ;)
 
lords said:
sorry to barge in on this subject ,as you know keep up with this forum and this is great info!
on the subject of static. I thought a capacitive hat was also for this?

Or does this not apply to mobile?
Click to expand...

thats an interesting question.

The tip of the antenna is the highest voltage point. So during transmit, its possible to get arcing from the tip if running high power. And the capacitive hat is at this highest voltage point on the antenna. Arcing is at the small ends of the rods. If there is a way to eliminate the sharp tips, then arcing shouldn't be a problem.
I also works in reverse for receive. if there is static electricity in the air, the antenna tip will collect it, as you drive along, and raise your noise floor.
 
QRN said:
Even with 20 turns of 6 ga. 12 inches in diameter I don't think it will be self resonant on 80m.Mobile antennas on 80m need a lot more turns than you think. Do you have an antenna analyzer? It will save a lot of headaches if you do.

I did make an antenna for mobile use and run it with my FC-40 auto tuner. It is self resonant on 15m. It consists of a 48 inch fiberglass bottom section 1/2 inch at the bottom and 3/8 inch at the top. It was a quad spreader from a Wilson Shooting Star. It is wound with the tinned braid from RG-8 coax at approx. 1 turn every 3/4 inch. Everything is covered in one continous length of black heat shrink tubing.I finished up the ends to accept standard 3/8-24 male threads on the bottom and female on the top.I used 3/8 antenna mounts that I modified a bit. Next I used a 6 inch fiberglass insulator with 3/8 threads on the bottom and a whip adapter on top. It came from an old Radio Shack CB antenna.I wound it with the braid similar to the bottom section.I used the braid so as to reduce losses as much as possible. Wire would work but would have higher losses due to the smaller size. On the very top I use a 58 inch whip from the old CB antenna.The whole thing is 9 foot 4 inches high and is mounted about four feet off the ground on my Ford Escape.It works GREAT on 20m and up,not too bad on 40m but summer time use on 80m has not been too good. I will wait and see what the winter brings. I used the 6 inch insulator BTW so I could change it out with various coil arrangments and see what works better. My experimental time however has been somewhat limited lately so I have just left it as is.The one thing is that the bottom section is VERY stiff while the top whip lays back a lot at highway speeds but the flexibility helps out when it hits the tree branches in town. ;)
Click to expand...

So for finding the high voltage point on the radiator for the purpose of locating the cap hat, I could maybe use a fluorescent tube next to it while on TX?

I had an analyzer but it "dun got taked" :cry:
 
Have a great time with your project! :D I've tinkered my way around many a problem; won a few, lost a few!!!

Ten or so years ago, a buddy of mine bought two Texas Bugcatchers; one a huge monster of 12" dia for 160 Meters, and another 6" one for 75 thru 10 Meters. Here's what happened. We got the 12 incher on 160 with a large cap hat located about 12+ inches above the coil since it isn't practical or feasible to locate hats at the top where they belong. The biggest trouble with 160 job was, with 500 watts, CORONA!!!
This is the "fire", or static, referred to above. A smoothe static ball helps, BUT what we don't think about is, the ball strikes things as it whips about, and it takes just one little "ding" or nick to make a place for corona to start. And if conditions are right, it is there even with the perfectly round static ball! :(
Now the power freaks and non-technical types that think about what a show they can put on (like putting neon bulbs on their whips thinking they are sooooooo smart) would be disappointed to learn that corona is NOT a good thing! It throws the antenna WAY off resonance, the SWR goes thru the ROOF, and it AIN'T good for the equipment! When it starts
shooting blue fire off the tip of the antenna..........
STOP!!!!!!!!!!!!!!!

We tuned up the 6" dia 80 Meter coil, got it perfect. Drove forward FIVE FEET and it was OUT of resonance!!!!!
:shock: Tuned it again in another spot. Got it perfect. Moved a few feet, and OUT again. This was because the "Q" of the coil was so HIGH, the bandwidth so NARROW, that it effectively cancelled everything we did!! So long as you tuned the thing and sat STILL, man.........................you couldn't ASK for a better "mobile" station!

Truly, I might be wrong, but I suspect this is what's gonna happen with your huge coil. However, I would LOVE to see this, AND be proven wrong. So have a ball with this, and come back and tell us your results when you're done!

Happy experimentin' ! That's what ham radio is really about after all!!!!

73

CWM
 
CDX-007 said:
So for finding the high voltage point on the radiator for the purpose of locating the cap hat, I could maybe use a fluorescent tube next to it while on TX?
Click to expand...

I can tell you where the HV point will be. At the very top.For antennas 1/4 wavelength and less, the highest voltage developed will at the high impedance point and that will be at the very top end. For antennas over 1/4 wavelength it will be somewhere below the top.Chances are you will not be using a mobile antenna over 66 feet high are you? ;)
 
QRN said:
CDX-007 said:
So for finding the high voltage point on the radiator for the purpose of locating the cap hat, I could maybe use a fluorescent tube next to it while on TX?
Click to expand...

I can tell you where the HV point will be. At the very top.For antennas 1/4 wavelength and less, the highest voltage developed will at the high impedance point and that will be at the very top end. For antennas over 1/4 wavelength it will be somewhere below the top.Chances are you will not be using a mobile antenna over 66 feet high are you? ;)
Click to expand...


OK, good points...

Sometimes it pays to think... of course the high voltage will be at the top, it's a friggen loaded QUARTER WAVE!! Thanks for that reminder.

So now I'm thinking a cap hat that big at the top would help eliminate corona but be a bugger to physically stabilize.

...and I thought the larger coil would have a lower Q but now you've got me preparing myself for the worst.

I guess if I needed to BBQ some skewered chicken on the way to a Ham picnic...

Actually, that corona discharge cuold be fun if I build the 4-400C amp I'm thinking about for the mobile.
It could prolly take it... for a while...

I went to buy some 1" fiberglass rod today but they only had 3/4" so I have a paid order waiting,

then,


















CORONA, BABY! HAH HAH HAH!! :twisted:


























.
 

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