hi
ive instaled a uhf in my disco , but no aerial as of yet. whats the difference between 4.5 &6 db gain? which is better?
i have no clue what one to purchase, thanks

The difference is that the 6db aerial concentrates the radiation pattern into a flatter shape than does the 4.5db one. This affects both transmission and reception, and will give better range provided the other station is neither much above nor below the aerial and provided the vehicle is on the level.

The practical difference is rarely very large, but the higher gain one is usually better all round in flat country, but the lower gain will be the better one in hilly terrain where it is more likely that the stations will be at different levels and that the vehicle will be on a slope.

John

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JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol

Yup, low gain for the hills and high gain for the plains.

Also consider the construction of the aerial. Fibreglass are more durable than wire, but wire is easier to mount and see through.

Here in the hills I have a 1/4 wave antenna (about 100mm long or 10 cm in dress making terms) which is mounted through the roof. Fantastic for hilly country and I have never had to replace it from a tree wiping it off. About $10 to buy. I also have no problems out on the flat land, but if you were to do a comparison the higher gain antenna would reach futher than the 1/4 wave.
Generally the lower the gain the fatter the signal but not as far. The higher the gain the more elongated the signal. Refer the the excellent diagram posted before this post.
cheers
Gillie.

A 3 to 4dB+ elevated feed antenna is generally a good allround choice. I keep 3 antennas based on where I am going. Long flat runs a 6dB or 4.5dB elevated feed, hilly or bushy areas, 4.5 elevated or a 3dB wire.

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If you get a ground plane dependant antenna, how you mount it will probably be more significant than whether its a 4.5 or 6 dB - center of the roof is best with this type. Doesnt matter so much with a ground plane independant antenna but higher is always better in either case

My preference these days is a magnetic roof mount, placed in the center of the roof and a $20 4.5 dB stainless steel whip, which is just about indestructable. It takes about 10 seconds to fit a $10 unity gain rubber whip for really hilly country.

Magnetic bases dont work so well on certain vehicles with aluminum roofs however........

Can both types of antanae be fitted/connected to deal with flat and hilly situation?

They can be fitted but not connected at the same time.

John

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JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol

As said you can just screw off one whip and put in another.

I have both but quite frankly I just leave the 1/4 wave ( unity gain ) on all the time now. The higher gain one just keeps hitting things.

Good to see that the lower figures really shine here, and as an old ham the unity to 4.5db was the most popular depending on your environment etc. When we look back to the 6db to 9db claims with monster antenna cases to suit, maybe big was not necessarily better.

I used to repair a few broken ones monsters for friends and the disguest they displayed when I showed them the insides as being just a series of 1/4 wave cut lengths of average quality coax. Mind you, nothing wrong with these feeds either, I guess they just wanted something shiny to make them feel better for all the money they spent!

By the way, dont forget the RFI base loaded either by direct coax feed types or a so239 add on as used everywhere in the commercial world are not generally any dearer than and can take a hiding.

Geoff