2M antenna question for serious HAM geeks...

[Mark W]

I'm hunting around for the ideal (for my uses) 2M antenna for use on my '40. I came across something interesting today while searching the net.

http://www.ac4rc.org/2M CPFL Antenna.pdf


The pluses that I see of this antenna are it's omni directional radiation, its lack of need for a ground plane, its efficient output, wide bandwidth, and short height.

For most people the boxy shape would be a none starter, but for most of my uses this sturdy, boxy 12.5 inch high setup will work better than a taller traditional mobile antenna. For most of the places I use the rig any branches that might overhang the rig are small and flexible enough that this antenna would brush them out of the way. A clip on mount of some sort that will break free but be quickly replaceable would address those times when we do encounter larger stuff but forget to remove the antenna.

I figure that if it was mounted in front of a roof rack on top of a '40 it would not stick up much, if any higher than the load n the rack.

Plus it would be rock simple and dirt cheap to make one to see if I it really works for me or not.


Now... for the radio gurus out there... What am I missing here? Anything important?

And... can I paint an antenna like this without degrading its performance?

Beyond preferring to have it black or some other solid color I'm not real worried about cosmetic tastes. Function over form ya know.


Mark...

 

[Kofoed]

Actually, the article says it is more isotropic with a circular polarization. Circular polarization in the upward direction is typical for satellite work. The broad side gain is 1.7 dbi, with 3.3 dbi front-to-back. A dipole is 2.1 dbi, and your existing 5/8 vertical is about 5.5dbi, omni. You will be giving up a lot of effective radiated power using this antenna.

Go here:

http://www.hamstick.com/hw-1.htm

http://www.angelfire.com/biz/AntVentures/#MOBANTS

You can either cut a hole in the roof or bolt it to the side of the top. Don't rule out 1/2 wave verticals
I have a 1/2 wave mounted on the windshield corner. The 1/2 wave length is the way to go. I

 

[Cruiserdrew]

I'm hunting around for the ideal (for my uses) 2M antenna for use on my '40. I came across something interesting today while searching the net.



I figure that if it was mounted in front of a roof rack on top of a '40 it would not stick up much, if any higher than the load n the rack.

Plus it would be rock simple and dirt cheap to make one to see if I it really works for me or not.


Now... for the radio gurus out there... What am I missing here? Anything important?


Mark...

Mark-I'm no radio guru, but I did just pass my general exam this weekend!

Anyway, I have been mounting and remounting antennas on my 40 this past week and have a new current "best" solution.

The 40 is hard, because of the lack of a metal roof, or even much metal up high where there would be a suitable counter poise.

For most uses a 5/8 wave antenna would be best, but without a counterpoise, you give up all of the gain and most of the SWR. Same with 1/4 wave antennas which are short on gain anyway.

To make a long story short, a 1/2 wave antenna could solve your problems. It is independent of a ground plane since the coax acts as the other 1/2 wave. Thus you can tune for very low SWR and not worry about the amount of available metal to reflect the signal off of.

Radial-Larsen makes very nice and very compact steel whip antennas. They are also very cheap, and the whip itself can be replaced for $12. They also make a calibrated spring that you can add to the base which makes the whole unit insanely flexible and unlikely to be damaged by trees, since it will just fold over. You use the cut chart and your SWR meter to clip the antenna to the right length, and off you go! I was using it this weekend with good performance to the bay area (90 miles), Grass Valley (70 miles) etc.

Anyway, if you used something like this, you could just build a simple mount on the rear rack to get the base of the antenna up as high as you could with practical considerations in mind.

Also, the Antenna with the NMO connector is only $36, and the spring is another $10. Just something to consider.

 

[Mark W]

I'm kinda focuses on the idea of putting heavy metal foil on the underside of the fiberglass roof and then grounding it to turn it into a counterpoise. Then I could stick with the 5/8 wave and it would work (*I think*) pretty effectively.

Unless someone who knows this stuff better than I do can show me that it's a non-starter, I'll probably give it a try soon.


Mark...

 

[e9999]

have a look at the SWR numbers I posted elsewhere... sobering... and that was an 80 with metal roof

 

[Kofoed]

I'm kinda focuses on the idea of putting heavy metal foil on the underside of the fiberglass roof and then grounding it to turn it into a counterpoise. Then I could stick with the 5/8 wave and it would work (*I think*) pretty effectively.

Unless someone who knows this stuff better than I do can show me that it's a non-starter, I'll probably give it a try soon.


Mark...

The 5/8 has more gain at the horizon than a 1/2 wave. You can compare some of the specs from antenna manufacturers sites, but, everything being equal it is a close to 0.4dB. This is not much.

Second, the need for a counterpoise to allow the antenna work electrically effecient requires that the area of the counterpoise (or length of radials) be a minimum fraction of a wavelength. The cookbook length is the same wave length as the vertical...maybe for an RV

Some web sources I found state that 1/8 wave length radials (diameter of counterpoise sheeting) are all that is required. I guess you can "go big" , which brings us back to the first paragraph.

It will probably work out fine with the 5/8 in any event.

 

[Cruiserdrew]

If you want to experiment, instead of sheeting, you could use 4-6 radial wires bonded to the roof fiberglass. That would be easier to handle than the sheet. Then if you made them as long as possible, like to the edges and corners of the fiberglass cap, you probably could get decent SWR and a good radiation pattern.

I'm thinking of a small copper disk, with an NMO sized hole in the center (3/4 inch), with the radials bonded to that. You could even use braid to make the radials. At 2m frequencies, I think the grounding through the coax would be pretty close to sufficient, or at least worth a try. The whole thing could be expoxied to the fiberglass.

The only downside would be the height above ground. Figure a huge SOA 40, and a 5/8 wave antenna, and all the trees/alders/brush of back country AK, and you would risk snapping off the antenna. If I were in AK, I'd make a mount on the rear rack just higher than the fuel cans, and run 1/2 wave whip. The radiation pattern would not quite as good, especially to the front, but the antenna would be protected and less likely to break off.

I will look forward to following what you come up with, and if you could post some SWR #s and pics of the final product.

 

[FJ40_owner]

If you want to experiment, instead of sheeting, you could use 4-6 radial wires bonded to the roof fiberglass. That would be easier to handle than the sheet. Then if you made them as long as possible, like to the edges and corners of the fiberglass cap, you probably could get decent SWR and a good radiation pattern.

I'm thinking of a small copper disk, with an NMO sized hole in the center (3/4 inch), with the radials bonded to that. You could even use braid to make the radials. At 2m frequencies, I think the grounding through the coax would be pretty close to sufficient, or at least worth a try. The whole thing could be expoxied to the fiberglass.

The only downside would be the height above ground. Figure a huge SOA 40, and a 5/8 wave antenna, and all the trees/alders/brush of back country AK, and you would risk snapping off the antenna. If I were in AK, I'd make a mount on the rear rack just higher than the fuel cans, and run 1/2 wave whip. The radiation pattern would not quite as good, especially to the front, but the antenna would be protected and less likely to break off.

I will look forward to following what you come up with, and if you could post some SWR #s and pics of the final product.

For the ability to hear weak signals I will always opt for a 5/8 over any antenna. There are many creative methods to have a groundplane. 4 radials of 19 inch metal, copper, wire, sheet metal should work fine. Look at some of the stuff Larsen offers. Very similar.

A half wave is a good compromise and a good solution in that it is slightly more broadbanded, but in my experience the 5/8 works for me. It hears better, and I am no Alligator... On all my vehicles.

My older dual banders have the luxury of two antenna ports. Therefore I am able to run a 5/8 on both bands.

Newer radios offer only one port requiring a dual band antenna, again a compromise.

In my spare time I would like to determine if running a newer radio with one port into a duplexer with two full 5/8's would be worth the effort. I wonder how much loss accross the duplexer there would be?

Probably not worth the expense of the duplexer and the additional antenna.

73's de N6KML.

 

[D'Animal]

Mark W

What did you end up doing and are you happy with it?

Did you make the big tinfoil hat for your rig or go with the radials?

 

[rusty_tlc]

If you want to experiment, instead of sheeting, you could use 4-6 radial wires bonded to the roof fiberglass. That would be easier to handle than the sheet. Then if you made them as long as possible, like to the edges and corners of the fiberglass cap, you probably could get decent SWR and a good radiation pattern.

I'm thinking of a small copper disk, with an NMO sized hole in the center (3/4 inch), with the radials bonded to that. You could even use braid to make the radials. At 2m frequencies, I think the grounding through the coax would be pretty close to sufficient, or at least worth a try. The whole thing could be expoxied to the fiberglass.

The only downside would be the height above ground. Figure a huge SOA 40, and a 5/8 wave antenna, and all the trees/alders/brush of back country AK, and you would risk snapping off the antenna. If I were in AK, I'd make a mount on the rear rack just higher than the fuel cans, and run 1/2 wave whip. The radiation pattern would not quite as good, especially to the front, but the antenna would be protected and less likely to break off.

I will look forward to following what you come up with, and if you could post some SWR #s and pics of the final product.

According to the ARRL antenna book 1/4 wave is the optimal size for a ground "plane". Radial wires work as well as a solid sheet of metal. For an FJ40 fiberglass roof I'd try to find some copper screen. Just cut a circle or square 1/2M in diameter and stick it up there somehow. McMaster Carr sells copper wire screen IIRC.

BTW a 5/8 wave should work pretty well without a ground plane, a 1/4 wave works with out a ground plane but the impedance match is horrible.