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

Sun Mar 11 04:41:54 EDT 2018

Andy,

Fine.  You do no need to wade thru two foot deep snow in the middle 
of night to read the meter.  I was talking about a resonant load, not 
the radiator.  Besides my RF ammeter is part of the amplifier which 
is not at the antenna.  Commercial stations have a transmitter hut at 
base of antenna and typically use a STL from the station.

My station is 120-foot from base of antenna so I run RF out to the 
antenna which is resonated using a base coil.

73, Ed - KL7UW

At 06:54 PM 3/10/2018, W2xj wrote:
>The proper place for an ammeter is the tower base. This is how AM 
>broadcast is required to be measure. BTW AM radiators are almost 
>never resonant.
>
>Sent from my iPad
>
> > On Mar 10, 2018, at 7:39 PM, Edward R Cole <kl7uw at acsalaska.net> wrote:
> >
> > 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
> > Dubus-NA Business mail:
> >  dubususa at gmail.com
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73, Ed - KL7UW
   http://www.kl7uw.com
Dubus-NA Business mail:
   dubususa at gmail.com 