? Silly question ?

[2E0PSX]

Hi all,

If my tx puts out 100 watts, through an atu (swr 4:1 before tuning, 1:1.5 after tuning), what would the likely power at the antenna ? I have a gut feeling that it is unlikely to be the true 100 watts tx'd but a lesser figure. Ignoring any feeder losses.

1: is there a quick way of working it out ?

In anticipation,

Phil

 

[nd5y]

See the links I posted at http://forums.radioreference.com/am...sion/252365-vswr-calculation.html#post1846647

 

[T04KLH9PW7AN]

Inline power meter at the base of the antenna, if you can get to it, would be the most accurate.

 

[Ed_Seedhouse]

According to the page at VSWR an SWR of 1.5 to 1 results in about a 0.18db loss.

 

[AgentCOPP1]

Inline power meter at the base of the antenna, if you can get to it, would be the most accurate.

That would only be accurate in the sense that it would tell you how much power is being lost in the feed line. The SWR reading would still be roughly the same because there will still be a small impedance mismatch in the line coming from the meter going to the antenna.

 

[zz0468]

According to the page at VSWR an SWR of 1.5 to 1 results in about a 0.18db loss.

Except that the SWR between the tuner and the antenna isn't changing. There will also be losses in the tuner itself.

 

[zz0468]

If my tx puts out 100 watts, through an atu (swr 4:1 before tuning, 1:1.5 after tuning), what would the likely power at the antenna ? I have a gut feeling that it is unlikely to be the true 100 watts tx'd but a lesser figure. Ignoring any feeder losses.

But you can't ignore feed line losses. That 4:1 swr will cause additional losses within the feed line, and will be the bulk of any power loss.

1: is there a quick way of working it out ?

Not with the information given.

What's missing is the feedline type, and things like the design of the tuner, q of the components in the tuner, frequencies, etc. etc. etc.

And then, there isn't really a quick way of determining it. What you can do is find charts and calculators to determine additional loss caused by swr on various types of coax at various frequencies.

Try this one: Coax Loss Calculator

The tuner itself will have some additional insertion loss, and that will be determined by what the precise matching solution is for that particular set of circumstances.

 

[jhooten]

That would only be accurate in the sense that it would tell you how much power is being lost in the feed line. The SWR reading would still be roughly the same because there will still be a small impedance mismatch in the line coming from the meter going to the antenna.

Double male connector, no jumper.

 

[AgentCOPP1]

Double male connector, no jumper.

I've never really heard of anyone connecting an SWR meter to an antenna without any jumper cable. Even then, connectors have their own characteristic impedance.

 

[jhooten]

It is really fun when you are 200 feet up the tower and realize your readers are on the dash of the truck. Ns are constant impedance.

Little more information on how we did it.
Power meter and dummy load on the antenna end of the cable on the ground to make sure the cable was good
Pull it up the tower and secure it.
Power meter and dummy load to compare the readings verify the cable wasn't damaged during the install.
Remove the dummy load and connect to the the antenna feed point check forward and reflected to make sure the antenna wasn't damaged on its way up.
Power meter to forward and set the transmitter output power to get the proper allowable system ERP.
Remove power meter connect cable to antenna and seal it with the weather seal kit.

And today, be glad you are no longer the tower monkey.