[600MRG] RF Ammeter Placement and Antenna Feedpoint Location

Sun Mar 11 19:03:36 EDT 2018

 Andy wrote:

>Where should an RF Ammeter be placed in the circuit ? My setup is: TX - Loading Coil - Inverted " L "
>Is the correct location for the meter on the TX side, or the Antenna side of the Loading Coil ? ( Coil is inside the shack ).

 The ammeter can be placed in series with the antenna/transmission line system at any point, but to make a meaningful measurement you have to know the radiation resistance of the antenna *at that point*.  A thermocouple ammeter or similar device responds to current that is drawn by the R part of the complex impedance R+jX and ignores the X (reactance).

For example, lets say you were measuring the current into purely resistive dummy load - impedance is 50-j0 ohms.  If the ammeter reads 2 amps, the power is 200 watts.  The formula is P = I-squared times R... in this case 2-squared times 50. 

Now lets suppose the dummy load still had a resistance of 50 ohms, but the wires are long, or the resistor is a wirewound coil.  Just making up numbers, say the impedance is 50+j35 ohms.  The ammeter would still read 2 amps, and the power would still be 200 watts.  

Now to consider a real antenna.  Let's say you calculate or measure that the radiator of your antenna has an impedance of 4-j277 ohms.  If the ammeter reads 2 amps when placed in series with the input of the radiator then the power in the antenna is 2-squared times 4 or 16 watts.  

If you moved the ammeter to be ahead of the loading coil or antenna matching device the impedance at that point would be difference, and quite likely the R would be different as well.  Since the rules in this case are interested in *radiated* power, I recommend placing the ammeter at the antenna radiator input (after the matching network) so that the resistive losses in the matching devices aren't included in the R.  But you could certainly measure or calculate the impedance at the input of the matching network and put the ammeter there and work out the power.  

Steve WD8DAS

sbjohnston at aol.com
http://www.wd8das.net/
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-----Original Message-----
From: Andy - KU4XR via 600MRG <600mrg at mailman.qth.net>
To: 600 meter group <600mrg at mailman.qth.net>
Sent: Sat, Mar 10, 2018 1:54 pm
Subject: [600MRG] RF Ammeter Placement and Antenna Feedpoint Location

Hi all:

I need to ask a couple of questions so I will know whether I am thinking correctly or not.

Question - 1: Where should an RF Ammeter be placed in the circuit ? My setup is: TX - Loading Coil - Inverted " L "
Is the correct location for the meter on the TX side, or the Antenna side of the Loading Coil ? ( Coil is inside the shack ).

Question - 2 : In my setup; What is the " Antenna Feedpoint " location ? TX end of the coil, or the Antenna end ?

Here is why I ask. Today I ran across a document for an MFJ Balanced Line Dual Current Meter .. In the document,
I read " How to calculate Antenna Feedpoint Impedance " .. It stated; " Divide applied power by square of Feedpoint Current. "
Curiousity took over and I came up with these readings .. TX power is 20 watts .. Using ammeter readings as they are ..
Antenna is - 65 feet of RG-59, shorted at both ends - connected to top of coil located in the shack - running a few feet
across the wall to the window, out to the mast at 30 feet AGL,  and a total length of 961 feet of Top Loading..
Top Loading is 4 legs - fanned out - connected together with a perimeter wire to make up the total length of the Top Loading..

Meter on antenna side of coil - 1.2 amps - squared is 1.44 amps ; 20 / 1.44 = 13.888 Ohms

Meter on the TX side of coil - .6 amps - squared is .36 amps ; 20 / .36 = 55.555 Ohms

The PA FET gets hot during TX - I have been told that my antenna is " close enough for the TX to operate into , but , 
not resonant enough to present a 50 ohm - non-reactive load  to the TX. ". Without test equipment; can I get it closer ?
I am not up to speed on antennas at MF, nor the heavy math for calculations.. 
My " assumption " is that my antenna is Capacitively Reactive ( have no instrument to measure it to find out )..
To obtain the readings I have - minimum SWR at maximum RF Current - I need 30 microHenries of coil Inductance.. 

Question - 3 : Could I possibly need an amount of Series Capacitance ( since the total length is 1/2 wave at 456 KHz ).
Can that Capacitance be calculated ?

The antenna is " not there yet " , and I hope I am not chasing - skinny bunnies around the bushes .. 

Thanks for reading, and any possible replies .. critique what is needed ( I know the radiating stuff should be outside ),
And post it to the list for the benefit of others .. Who knows; someone might learn something from my escapade's ..

73: Andy - KU4XR

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