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#31
May 18th 04, 08:45 PM
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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 |
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