Antenna Theory

[scubadude85]

Please help a noob out.

 

1) I understand SWR needs to be as low as you can get.

 

2) dB I don't understand are low numbers good or high numbers?

 

3) When installing an outdoor antenna and weather packing the connectors can one use die-electric grease or just heat shrink tubing?

[kidphc]

Low swr is better. 1:1 means you are getting all the power out. Higher means you are getting reflected power or wasting some as heat. There is an old video (50's 60's) that explains it with models. Swr is not the end all to antennas and many focus on it too much. But generally strive for lower.

 

Db is the increase of gain on the antenna. Dbi means it is based off a reference. Usually DBi gives you a better idea of increase in gain to a reference antenna. Where db may the gain vs a pencil for all you know. So 3 db with make your 5 watt radio equal to a radio with 15 watts of power with no gain.

 

The higher the gain value normally the radiation pattern is flatter with a higher take off angle. For example 1/4 wave antenna is a bit more spherical (like a ball). So when offroading it can be beneficial if you are going up a dune at the crest and there is a truck below you, you'll can hear him. With a half wave design the pattern is flatter and you may not hear him.

 

However, when using a repeater from a mobile the 1/2 wave can more beneficial because then repeater is higher and you have more gain.

 

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[kidphc]

Sorry the third part. You can use a sealant for the connectors. It is available from most radio shops. I'd use a weatherproof boot specific to the connector and maybe some electrical tape.

 

Problem with heat shrink is if you need to remove the wrap to service the connector you need to cut it off. In doing so you may cut the cable.

 

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[kidphc]

Here is the video on swr (standing wave ratio).

 

Long and reminds of video from grade school. But worth it.

 

 

 

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[Lscott]

You may find this book of interest. The book is about antennas and transmission lines. I hope Firefox didn't mangle this link again.

 

http://www.w3pga.org/Antenna Books/Reflections III.pdf

 

It's a good reference to keep on hand.

[Lscott]

You may find this book of interest. The book is about antennas and transmission lines. I hope Firefox didn't mangle this link again.

 

http://www.w3pga.org/Antenna Books/Reflections III.pdf

 

It's a good reference to keep on hand.

If the link above doesn't work, you have to copy the whole line and past it into the browser. Then try this one.

 

http://www.w3pga.org/Antenna%20Books/Reflections%20III.pdf