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Old 11-06-2009, 19:56   #1
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VHF Base Station

I am putting a VHF base station in my home, Grenada. I am getting ready to mount an 8’ antenna and have two places that I may pick from. The highest has a 2.148 db loss and is 3 feet higher than the next alternative; the second has a 1.401 loss. The antenna is rated at 6db. These numbers came from a cable calculator on the Internet for cable length.
What does this mean and is it significant? I am guessing that the higher the better in this instance
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Old 11-06-2009, 20:32   #2
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db refers to decibel. It's a commonly used measure of power. 3db = 2 X power, 6db = 4 X power, etc. Example: a 25 watt transmitter with a 6db gain antenna would have an effective radiated power of 25 X 4 or 100 watts, not counting losses in the transmission line, etc.

At VHF frequencies, coax loss is important if the coax is very long. It can be greatly reduced by using proper coax. RG-213 or 214 is preferred and commonly available (better than RG8 and much better than the thinner RG58 or 8X). LM400 and other exotic varieties of coax have even lower losses. You can get an idea of what types of coax are available, and their properties, from the following: RF Connection Coaxial Cable

Generally, mounting the antenna higher is better. But, I doubt that 3' would make much difference at all. Put the antenna in the clear as best you can.

Assume the govt. of Grenada is cool with this? It's a hassle getting a shore station license in the U.S., but can be done if you have a clear commercial need.

Bill
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Old 11-06-2009, 20:40   #3
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Well, 3 dB loss means that only half the power from the radio reaches the antenna and 6 dB loss would mean only 25% reaches the antenna. So for the highest location you loose about 33% which is a lot. I would advise you to use a better cable. What are the distances we're talking about? Use that calculator for RG8U cable and RG213 cable and see what you get.

I did the same for a friend of mine in Grenada (Further east on the south coast) and I could reach him from more than 40 nm south of him. He was on a decent hill so height was not an issue.

ciao!
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Old 11-06-2009, 20:43   #4
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Quote:
Originally Posted by btrayfors View Post
Assume the govt. of Grenada is cool with this? It's a hassle getting a shore station license in the U.S., but can be done if you have a clear commercial need.
Bill,

I don't think these licenses exist in Grenada or most of the other islands in the east Carieb. Even the taxi's have VHF, supermarkets advertise their daily specials etc.

ciao!
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Old 11-06-2009, 22:29   #5
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Agreed, VHF's in many overseas countries are used like they are a powerful CB radio. You might as well get an illegal one (illegal in the US) that transmits more than 25 watts....that's only if nobody is going to care.
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Old 12-06-2009, 07:44   #6
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This is what I think I have:
ICOM 422 @ 25 Watts Power in To buy
RG213 cable To buy
Shakespear 6db Antenna have
FreQ @ 16 and 68
SWR 1.3
Cable run is 26 ft & 40 ft
With the internet calculation I have a reduction of .459 @ 26 ft and a reduction of .706 @ 40 ft. This from 6db times 25 watts is 138.52 watts @ 26 ft and 132.35 @ 40 ft. I am guessing that either spot for my purposes will do fine.
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Old 12-06-2009, 09:06   #7
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Revised numbers...

This is what I think I have:
ICOM 422 @ 25 Watts Power in To buy
RG213 cable To buy
Shakespear 6db Antenna have
FreQ @ 16 and 68
SWR 1.3
Cable run is 26 ft & 40 ft
With the internet calculation I have a reduction of .727 @ 26 ft and a reduction of 1.116 @ 40 ft. This from 6db times 25 watts is 131.92 watts @ 26 ft and 122.10 @ 40 ft. (channel 16). I am guessing that either spot for my purposes will do fine.
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Old 12-06-2009, 11:00   #8
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VHF calculations

Quote:
Originally Posted by landonshaw View Post
I am putting a VHF base station in my home, Grenada. I am getting ready to mount an 8’ antenna and have two places that I may pick from. The highest has a 2.148 db loss and is 3 feet higher than the next alternative; the second has a 1.401 loss. The antenna is rated at 6db. These numbers came from a cable calculator on the Internet for cable length.
What does this mean and is it significant? I am guessing that the higher the better in this instance
A friend of mine is an electronic engineer who loves to do calculations for such things. If the other posts don't help you, message me & I'll pass your question along to him.
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Old 12-06-2009, 16:29   #9
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What you are trying is to calculate ERP. When you feed 25W into a 6 dB gain antenna you get 100W ERP (each 3 dB doubles).

I wonder what you mean with "reduction". Are those cable + connector insetion losses in dB? If so you shouldn't worry. The RG213 is perfect for VHF.

The 2 locations for the antenna: see how much clearance you have around the antenna in these spot. Also look for obstructions in the directions you want your signal to travel. Choose the location on these criteria instead of cable lengths.

ciao!
Nick.
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Old 12-06-2009, 17:26   #10
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Thank you for your help. Once back from the states with my base station I will install.
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Old 13-06-2009, 04:32   #11
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ERP (Effective or equivalent Radiated Power), as applied to antenna systems, is determined by subtracting system losses and adding system gains.
ERP takes into consideration transmitter power output , transmission line losses, connector losses, antenna directivity, mounting height, and etc.

Goto ➥ ERP Calculator Amateur Radio Software by KD4UDY
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