[600MRG] RF Ammeter Placement and Antenna Feedpoint Location - Extra Info

[Edward R Cole]

Andy,

I think traditional location of RF ammeter was at Tx output (which is 
50-ohm for most).  P= I^2*R  Z=R if antenna is resonant.

Probably handy to have in shack for monitoring output.

If used at antenna you would need to know R at the metering point to 
calculate P.  I measure Z = 20 +j0 at coil with my modified MFJ-269B.

Broadcast stations are on one fixed freq. so antenna is tuned for 
that and monitored in the station.  Our band is so narrow you can do 
the same as long as you realize the antenna will be resonant at only 
one freq (Unless you use a tuner device).

I am setting my antenna resonance at 473 or 474 KHz to cover 471-476 
KHz (SWR bw = 5 KHz).

I run 100w so my 50-ohm RF ammeter reads 1.40a.  EIRP approx 4w.

73, Ed - KL7UW
Got wires up in air this week so only need to set coil tap for 
resonance.  Been clearing two foot of new snow so had no time.

At 02:41 PM 3/10/2018, Andy - KU4XR via 600MRG wrote:
>Off list replies asked, so I will add this info .. The Loading Coil 
>is a Tappable Tank Coil.. I ground the end at the TX,
>and tap UP to match the TX to 50 ohms .. I tap DOWN from the top to 
>find the point where Minimum SWR and
>Maximum RF Current happens at the same time .. The Coil is in Series 
>with the Antenna / TX circuit ..
>No Shunt " C " .. I finally tested the RF Ammeter with my Dummy Load 
>.. I measured it with a meter at 51 ohms..
>I connected it to the TX and the current read .625 amps ( squared = 
>.390 ) - 20 watts / .390 ( I^2 ) = 51.282 ohms.
>At least I feel more confident that the RF Ammeter is trustable ..
>
>Andy - KU4XR
>__________________________________________________________________________________________________
>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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73, Ed - KL7UW
   http://www.kl7uw.com
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