Dan Richardson wrote in message . ..
On 18 May 2004 06:19:50 -0700, (Brian Kelly) wrote:

Then howcum my 5/8 wave mag mount 2m mobile antenna very significantly
outperforms the 1/4 wave mag mount antenna I used to use? Same ground
condx, same power, same feeder length, same vehicle, etc. I suspect
it's in the differences in the TO angles.


My previous response was for a ground plane antenna mounted above
ground and you are addressing a mobile installation. They are
different.

Agreed.

I've done modeling of 1/4,1/2 and 5/8-wave whips using several wire
grid models for vehicles (small and mid-sized car, small pickup truck
and a SUV). I found that the vehicle's size, shape and whip location
plays a major part in performance.

No surprise there although my instincts tell me that once some number
of "critical" square feet of vehicle sheet metal is "achieved" the
size of the vehicle has a less pronounced effect particulary at VHF
and UHF freqs. But modeling has the annoying ability to deflate
instincts.

I also noted, on the average, the 5/8 produce a slight gain over all,
but it was possible to find azimuth directions that a 5/8-wave would
produced almost 3 db gain over itself - depending what vehicle it was
mounted on.

That would easily explain my offhand experiences. In fact I did it
again last weekend. An buddy of mine is temporarily laid up in a
skilled care facility so I got his home 2M FM station running in his
room. I installed his 1/4 wave magmount whip on one of those typical
steel-shrouded HVAC units often found under the windows of patient
rooms. He couldn't hit the repeater. I went back later and installed
my 5/8 wave whip and yup, now he can hit the repeater.

The most outstanding feature I saw was a 5/8-wave whip azimuth pattern
was less influenced by the vehicle geometry.

The operation is more like a lop-sided dipole with the vehicle body
being on leg.

I don't feel you can accurately predict how a the whips will perform
on a vehicle based upon operation on another vehicle - unless both
vehicles and the antenna locations are the same.

I should add for all models the whips where placed top-dead-center of
the vehicle's roof.

100% agreed and it all fits. Tnx.

73,
Danny, K6MHE

Brian w3rv