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Old 08-10-2015, 22:22   #31
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Re: Defective VHF or Antenna?

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Originally Posted by ka4wja View Post
(and the voltage drops at the square of the current drawn, so twice the current is 4 times the voltage drop, etc.)
Sorry to be pedantic, but this isn't right. The voltage drop is linear relative to the current drawn. If your wires drop 0.1V at 1 A (for example), at 2A the drop will be 0.2V.

Now *power* changes as the square of current (or of voltage):
P = I(squared) * R.
(P = power, I = current, R = resistance)

Regardless, checking the voltage at the radio power connection is a great idea.
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Old 09-10-2015, 09:33   #32
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Re: Defective VHF or Antenna?

Paul is of course correct!
I've got one of those summer head colds!!
Sucking on a Cold-Ezze lozenge as I type this! (is it possible that all the fluid in the human body flows into your head??)


Just as an example 1 amp drawn thru 15' of #12 copper, at 12vdc would only drop 0.05vdc...but 6 amps drawn thru the same 15' of #12 copper (at 12vdc) would drop about 0.3vdc...but if you have a bad connection / corroded terminal, etc. this can drop a couple volts of more!


Fair winds!

John
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Old 09-10-2015, 12:20   #33
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Re: Defective VHF or Antenna?

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Originally Posted by transmitterdan View Post
Cadence,

I am unsure what you are asking. Try again and I will try to help.
I didn't think standing wave ratio meter was frequency sensitive. I guess, I can see the meter may be power sensitive?
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Old 09-10-2015, 13:31   #34
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Defective VHF or Antenna?

Still not sure what you are asking. In general, the standing wave will be sensitive to frequency. Antennas have different VSWR at different frequencies.

VSWR should not be sensitive to power. However the meter will require some minimum power to make an accurate reading.

Meters are designed to cover a certain frequency range. Outside that range the accuracy will not be good.
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Old 09-10-2015, 14:48   #35
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Re: Defective VHF or Antenna?

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Still not sure what you are asking. In general, the standing wave will be sensitive to frequency. Antennas have different VSWR at different frequencies.

VSWR should not be sensitive to power. However the meter will require some minimum power to make an accurate reading.

Meters are designed to cover a certain frequency range. Outside that range the accuracy will not be good.
I guess that answers it? Given that, they may rely a tank circuit for the measurement? As I recall tuning is related to the wave length and antenna length or some fraction there of that wave length. Not that I care to much. Just idle curiousity at this point. I good skip will get you anyplace.
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Old 09-10-2015, 19:03   #36
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Re: Defective VHF or Antenna?

Cadence,
Yes, "power" measurements are very frequency sensitive....
But, VSWR can be measured to a fair accuracy, using a meter out-of-frequency-range..

In theory the way many "CB-type" SWR meters are made (typically a simple stripline circuit), they will reproduce a reasonable accuracy of the actual VSWR, even at 156mhz...
Quote:
Originally Posted by Cadence View Post
I didn't think standing wave ratio meter was frequency sensitive. I guess, I can see the meter may be power sensitive?
So, as long as you are not concerned about absolute power measurements, they will work...to some extent!

But, some of these meters are quite sensitive and surprisingly this sensitivity actually increases in the upper HF and lower VHF ranges....so, if the meter is designed for low power (5-10w at 27mhz) you may not be able to adjust the fwd-power-calibration properly (full-scale), in order to get a decent reading when then switching to ref....

Also, there can be some imbalance between the fwd and ref sensing....further complicating matters...

So, in actual practice, you should be able to tell the difference between an antenna that has some serious issues (swr > 3:1) and one that is working well (swr ~ 1.5:1)...
But, doubtful that you'd be able to effectively tell whether the antenna had a 2.2:1 or 1.8:1 swr!!

BUT....


But, the big problem here in these situations is that many sailors are trying to measure the VSWR of their VHF antenna while testing at the transmitter end / going thru a fairly lossy transmission line....
No, nobody is saying that they have to climb the mast!!
Just wanting to point out that the REAL VSWR of the antenna is always going to be higher than what you measure at the transmitter end, due to feedline/transmission line losses...
At HF freqs, this is usually a moot point due to the low losses....but when line losses move above 2.5db to 3db (100' of RG-213 at 156mhz, etc.), the user needs to be aware of the inherent inaccuracy of antenna VSWR measurements when doing this at the transmitter end of the cable...



I do hope this helps...

Fair winds..

John
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Old 10-10-2015, 10:43   #37
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Re: Defective VHF or Antenna?

Quote:
Originally Posted by ka4wja View Post
Cadence,
Yes, "power" measurements are very frequency sensitive....
But, VSWR can be measured to a fair accuracy, using a meter out-of-frequency-range..

In theory the way many "CB-type" SWR meters are made (typically a simple stripline circuit), they will reproduce a reasonable accuracy of the actual VSWR, even at 156mhz... So, as long as you are not concerned about absolute power measurements, they will work...to some extent!

But, some of these meters are quite sensitive and surprisingly this sensitivity actually increases in the upper HF and lower VHF ranges....so, if the meter is designed for low power (5-10w at 27mhz) you may not be able to adjust the fwd-power-calibration properly (full-scale), in order to get a decent reading when then switching to ref....

Also, there can be some imbalance between the fwd and ref sensing....further complicating matters...

So, in actual practice, you should be able to tell the difference between an antenna that has some serious issues (swr > 3:1) and one that is working well (swr ~ 1.5:1)...
But, doubtful that you'd be able to effectively tell whether the antenna had a 2.2:1 or 1.8:1 swr!!

BUT....


But, the big problem here in these situations is that many sailors are trying to measure the VSWR of their VHF antenna while testing at the transmitter end / going thru a fairly lossy transmission line....
No, nobody is saying that they have to climb the mast!!
Just wanting to point out that the REAL VSWR of the antenna is always going to be higher than what you measure at the transmitter end, due to feedline/transmission line losses...
At HF freqs, this is usually a moot point due to the low losses....but when line losses move above 2.5db to 3db (100' of RG-213 at 156mhz, etc.), the user needs to be aware of the inherent inaccuracy of antenna VSWR measurements when doing this at the transmitter end of the cable...



I do hope this helps...

Fair winds..

John
Thanks a well written piece. Hopefully will help the OPer.
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Old 10-10-2015, 10:53   #38
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Re: Defective VHF or Antenna?

It's common to get a squeal with low voltage to the radio. Check the voltage at the radio, while receiving and transmitting on both low and high power. What are the 3 readings and how much of a drop on high?
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