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Old 02-09-2019, 10:37   #1
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VHF antenna amp.

I used the wrong coax cable up the mast (RG-58) so we have lousy radio. The antenna is new and the radio is an Icom 602. Now I've bought a 30db 1w 12.5vdc 50-500mhz linear amp. Does that sound about right or have I wasted more money?
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Old 02-09-2019, 10:41   #2
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Re: VHF antenna amp.

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I used the wrong coax cable up the mast (RG-58) so we have lousy radio. The antenna is new and the radio is an Icom 602. Now I've bought a 30db 1w 12.5vdc 50-500mhz linear amp. Does that sound about right or have I wasted more money?
Yes you wasted money. You need to replace the cable with something like RG-213 but also make sure that the connectors are attached properly. Most are not done correctly that I find when helping others.
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Old 02-09-2019, 11:40   #3
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Re: VHF antenna amp.

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Originally Posted by Ecos View Post
Does that sound about right or have I wasted more money?
Sounds illegal..... altering a type approved radio.....

Though rg58 isn't great but shouldn't be completely disastrous - maybe you've a badly made connector somewhere along the way,
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Old 02-09-2019, 12:15   #4
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Re: VHF antenna amp.

What do you describe as lousy? Have you used an SWR meter to evaluate your antenna system? What is the distance from the antenna to the radio? How many connectors do you have in the system? Is all of the coax new or just the part going up the mast? RG213 is great stuff but it can be hard to run in tight places. The Icom 602 hasn't been made for a while, so I assume it is not new. How do you know it's lousy cable and not just a bad radio? I'm assuming that you are hoping the 1 watt linear amplifier will boost your receive signal. What will happen to this 1 watt amplifier when your radio tries to stuff 25 watts into it? Do you know how to wire it into the radio so that its taken out of the circuit when you try to transmit? This last step as Conachair pointed out is probably quite illegal.
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Old 02-09-2019, 12:27   #5
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Re: VHF antenna amp.

Quote:
Originally Posted by Ecos View Post
I used the wrong coax cable up the mast (RG-58) so we have lousy radio. The antenna is new and the radio is an Icom 602. Now I've bought a 30db 1w 12.5vdc 50-500mhz linear amp. Does that sound about right or have I wasted more money?

RG-58, if installed correctly, should work OK. If it is indeed lousy then you have a problem with damage to the coax, improper terminations and connections, or problems in the antenna or the radio. You would want to check any connectors at the base of the mast.



It is extremely unusual for a broadband amplifier to help in these situations.
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Old 02-09-2019, 12:33   #6
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Re: VHF antenna amp.

Quote:
Originally Posted by Ecos View Post
I used the wrong coax cable up the mast (RG-58) so we have lousy radio. The antenna is new and the radio is an Icom 602. Now I've bought a 30db 1w 12.5vdc 50-500mhz linear amp. Does that sound about right or have I wasted more money?
What?

What is a “30dB 1 watt” amplifier?
That is meaningless garbage advertising.


You have a radio capable of 25 watts output - why would you want a 1watt amp?


There is no such thing as the wrong coax unless you use one of incorrect impedance. RG 58 has slightly more loss at VHF than larger coax but you won’t hear any difference except under very marginal conditions.
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Old 02-09-2019, 13:18   #7
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Re: VHF antenna amp.

Is been this way for 12 years. No better, no worse over the years. The radio was new at the that time and the antenna and all the connectors to the base of the mast were new last year. From the base of the mast to the radio is the 1970's coax which is a much larger cable than the rg-58. There is also a new connector at the radio. I have never tried an SWR meter.
The radio works just fine out in the open and up close, but during the morning cruiser net's we often don't hear everybody in the area.
This new amp has a peak power of 30w, so I guess it could handle the 25w from the radio. I don't understand the 30 db gain part. What does that mean?
As for being illegal, there is a boat here in the jungle with an amp and he sounds great.
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Old 02-09-2019, 14:11   #8
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Re: VHF antenna amp.

Quote:
Originally Posted by Ecos View Post
Is been this way for 12 years. No better, no worse over the years. The radio was new at the that time and the antenna and all the connectors to the base of the mast were new last year. From the base of the mast to the radio is the 1970's coax which is a much larger cable than the rg-58. There is also a new connector at the radio. I have never tried an SWR meter.
The radio works just fine out in the open and up close, but during the morning cruiser net's we often don't hear everybody in the area.
This new amp has a peak power of 30w, so I guess it could handle the 25w from the radio. I don't understand the 30 db gain part. What does that mean?
As for being illegal, there a boat here in the jungle with an amp and he sounds great.

Much bigger in diameter cable sounds like RG213. Your original description of the 1w 12.5v is what is confusing. That would seem to indicate 1 watt and if it's 30db that means that the input signal is amplified 1000 times. Putting even 1 full watt into the receiver circuit is an awful lot of power. On the transmit side a maximum output of 30watts (illegal in the states) doesn't mean diddly over the normal 25 watts. VHF is line of sight and if you're putting 25 watts out of the antenna then everyone in line of site will be able to hear you. An additional 5 watts won't make any difference. The guy with the amp probably has a clear line of sight to you, not to mention he might be using a higher gain antenna.



The "out in the open" and "up close" is an indicator of signal loss in your antenna system. I've had this happen twice. Once when the coax in the mast went bad, and once when a cold solder joint in the radio finally quit. It came from the factory that way and worked for 2 years until it didn't. Oddly enough it would work on low power but not high power. It transmitted fine at 1 watt but would not receive past 2 miles. When the coax went bad it was also good to about 2 miles. My suspicion is that one of the connectors has a bad solder joint, or if you used a penetrator type connector that a stray shield wire has come in contact with the core penetrator. It's easy enough for that to happen and it only takes one strand and you might as well not have an antenna.



If you tell us the brand and model amplifier perhaps we can give you a dependable opinion. Most of these amplifiers are designed for milliwatt inputs and if you put 25 watts from your transmitter through them there would probably be wisps of smoke emanating from your unit.



Find someone down there with an SWR meter that know how to use it and see if they'll help you out. Many, if not most, HAMs will have one and most are happy to share their knowledge. They can check out your antenna system and help you isolate the problem.
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Old 02-09-2019, 15:26   #9
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Re: VHF antenna amp.

Lots of very good advice upthread.

IMO, a SWR meter only tool needed to find exactly where the problem is.

This is probably the best advice by a long shot.
Quote:
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...............
Find someone down there with an SWR meter that know how to use it and see if they'll help you out. Many, if not most, HAMs will have one and most are happy to share their knowledge. They can check out your antenna system and help you isolate the problem.
Remember any antenna and coax defects will affect both transmit and receive side of the radio equally.

Trying the linear amp is analogous to having a cooling problem in an engine and adding a second water pump without doing anything else - might work but unlikely.
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Old 02-09-2019, 17:06   #10
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Re: VHF antenna amp.

With what you told us, you would be the first to not have bad connections as the main problem. Having new connectors does not mean it’s good... it is about how they are measured, stripped, crimped, soldered, aligned and tightened that matter.

I’ll also repeat my recommendation to replace the cable with RG-213 and not just the RG-58 part but the whole system from radio to antenna incl. all connectors regardless of them being from 1970 or new. Do it right for once and for all
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Old 03-09-2019, 05:00   #11
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Re: VHF antenna amp.

Quote:
Originally Posted by Ecos View Post
.............
This new amp has a peak power of 30w, so I guess it could handle the 25w from the radio. I don't understand the 30 db gain part. What does that mean?
..........
Technically 30 dB simply means an amplification factor of 1,000. I don't know what the actual specifications of this linear amplifier are or what the marketing guff says but it would be reasonable to presume they mean the amplifier can increase the input signal by a thousand times provided the output remains less than 30W. If so, for a full 30W output, the input must be no more than 30 mW.

FWIW, the dB scale is a logarithmic unit that indicates ratio or gain.
0 dB = ratio of 1
10 dB = ratio of 10
20 dB = ratio of 100
30 dB = ratio of 1,000
40 dB = ratio of 10,000
You get the idea.

Quote:
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..... I have never tried an SWR meter.
.....
Maybe some basic information about RF and what a Standing Wave Ratio (SWR) meter does could be helpful however please disregard this post if you already know this stuff.

A SWR meter really just directionally measures RF power. Why directionally; because the RF power from the radio can be in two directions - forward and reflected.

In the ideal world, all the power from the radio is forward - that is, it is generated in the radio and goes along the coax to the antenna where it is radiated into space. So in this ideal world, 25 W is generated from the radio and 25 W is radiated from the antenna.

In the real world, the coax isn't perfect and neither is the antenna. So some of the power from the radio is reflected back towards the transmitter every time there is a mis-match in the coax or the antenna. This is called reflected power and so reduces the amount of power that can be transmitted. Poor connectors and corroded coax are just some of the things that can cause such a mis-match.

The SWR meter can measure the forward and reflected RF power. In the ideal world, it would measure 25 W of forward power and 0 W of reflected power. In the real world, there might be say 15 W forward and 10 W reflected. Without going into a long explanation about the maths, this gives us a SWR of about 10 - which is really bad. If say the forward power measured 20 w and the reflected power was say 2 W, the SWR would be about 2 - which is only just OK.

The long and short of it all is the SWR meter can tell you how efficient the coax and antenna system really is and where any faults might be. It also tells you how much power is being generated by the radio and how much is getting to the antenna and what the antenna is doing with the power i.e. is it radiating it or is it reflecting it back towards the radio.

Please remember this is the simple explanation and not the truly technical one but it should give you the general idea of how the SWR meter can be useful.
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Old 03-09-2019, 05:23   #12
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Re: VHF antenna amp.

Further to my previous post, I forgot to say the SWR meter can also determine any losses in the coax etc.

For instance, your radio may be generating 25 W at the rear connector but by the time the RF gets to the antenna via a poor coax, there might be only 1 W (or less) of RF power left for the antenna to transmit.

1 W is plenty for close in or nearby contacts but it isn't going to get you much range.

In the RF world, the SWR meter is your best friend and 50 Ohm ideal (or dummy) load is your next best friend. The 50 Ohm replicates a perfect antenna etc and it used in conjunction with the SWR meter to determine what is going on in the transmit path of the radio/coax/antenna.
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Old 03-09-2019, 07:04   #13
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Re: VHF antenna amp.

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Originally Posted by Wotname View Post
.

The long and short of it all is the SWR meter can tell you how efficient the coax and antenna system really is ..
Just one point for clarification - an SWR meter doesn’t indicate antenna efficacy. For example, one could attach a light bulb to the end of the coax (or dummy load) and see a 1:1 SWR ratio.

An SWR meter ‘sees’ only what’s happening at a given point in the coax. An antenna analyzer can do what you suggest but that’s another animal entirely.

And there is no such thing as a 1watt, 30dB amplifier. This whole discussion is a fantasy.
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Old 03-09-2019, 09:47   #14
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Re: VHF antenna amp.

Thanks for patience in explaining this subject, interesting. I should maybe buy an swr meter.
I would need to pull the mast to change the wires and that is not in the plan.
This is hardly a fantasy, Ebay is full of so called 1w linear amps. I will check back one day and give an update
thanks
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Old 03-09-2019, 10:03   #15
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Re: VHF antenna amp.

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As for being illegal, there is a boat here in the jungle with an amp and he sounds great.
It's illegal for good reason, people not knowing what they are doing making a mess on the airwaves, for all you know the guy is swamping every signal on nearby frequencies. VHF radios are well designed already, if they need something else to work then something is up outside the radio. Better idea to fix that.
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