18" dual band (2m/23cm) image plane needed

[KA5UTN]

Hello;

I own the subject antenna and would like to mount it to my luggage rack front cross member on my 2003 4Runner.  The cross member is about 8 inches above the truck roof.  I know a better phsical image plane would significantly improve the Tx signal pattern and I'm willing to mount the antenna on a circular piece of stainless steel that will be mounted on the cross member.

Question:  What minimum diameter disk is recommended?

73s

Mark

[DROLLTROLL]

1/4 wavelength of the band you intend to transmit on. If it's 2 meters then your ground plane needs to be 0.5 meters. A square piece of metal would work just as well as a circular disc.

[W3LK]

18 to 20 inches will do just fine, although I am not sure you will see any measurable difference as long at the bar/rack is actually bonded to the body.

The disk would need to be fairly thick - at least 1/8" - to keep it from fluttering at highway speeds. I would also fasten the edges to the bar/rack.

73,

Lon - W3LK
Naugatuck, Connecticut

[K0BG]

Just about every neophyte amateur is looking for a way to circumvent drilling a hole and mounting their antenna correctly. It's the old conundrum; buy a decent antenna, and then mount it in the least efficient manner.

Yes, a flat plate will work, and in fact it will work to some degree if you do nothing in the way of a ground plane. But it isn't the radiation pattern that's skewed or reduced, it's the effective radiated power. Ground loss is just that, loss!

Alan, KØBG
www.k0bg.com

[AA4PB]

The ground plane would work best if it is at least 1/4 wavelength in all directions. That's 1/2 wavelength (or about 36 inches) in diameter with the antenna mounted in the center.

[KA5UTN]

Thank you all for your answers and inputs.  Yes, I suppose I do want it all: a good and even radiated pattern, a low profile antenna, and the convenient mounting point of my luggage rack front cross member.

No, the rack is not bonded to the vehicle as I can see rubber mounts at the four corners and side legs. Of course the 10 mounting bolts are not insulated from the chassis as far as I could see. I'll measure for continutiy to see for sure.

If I take the "no compromise" position of A0BG and mount the antenna through the roof, behind where the sun roof slides when open, what effect will my luggage rack have on the radiated pattern? Consider that the rack stands about 8 or 9 inches above the roof, would surround the antenna and would be well within 3 ft on all sides of the antenna (there are 4 cross members and I can slide them away only so far). While we're at it, let's consider that it is grounded to the chassis.

Also my thanks to A0BG.  I had the same reaction to being called a ham radio neophyte as I do when, at 46 years of age, I get carded by some 25 year old, when  trying to buy an adult beverage <big laugh>.  Thanks again Alan, and I like your website too.

Yes, Alan, I know... Mount the antenna through the roof and take my luggage rack off of the vehicle because nothing is more important than a good image plane and clean omidirectional radiation pattern <tongue-in-cheek grin>.

73s
Mark