I am interested in the best way to properly base load an antenna system for a Radio Systems 20 watt AM transmitter factory set at 650khz. The antenna impediance is 50 ohms. I would like to use a vertical radial (scwis: radiator?) of 98 inches.
I am interested in the best way to properly base load an antenna system for a Radio Systems 20 watt AM transmitter factory set at 650khz. The antenna impediance is 50 ohms. I would like to use a vertical radial (scwis: radiator?) of 98 inches.
I have used follows these steps.
First, calculate the vertical radiator capacitance using length, diameter and frequency in MHz (0.650 for you).
That formula looks like this:
Where C = capacitance in picoFarads
f = frequency in MHz, h = height of antenna in feet and
d = diameter in inches
Next calculate the inductance needed to match the transmitter to that capacitance at that frequency:
Where L = inductance in Henrys (not micro or milli)
and f = frequency in Hertz (not Mega)
and C = capacitance in Farads (not pico)
Next, calculate the number of turns needed for an air coil:
Where a = the radius of the form and b = the length of the coil and
L = inductance in Microhenrys
Finally, calculate the 50 Ohm tap point for the coil.
Calculate the inductive reactance:
Where f = frequency in Hertz (not Mega) and
Where L = inductance in Henrys
Calculate the impedance ratio for 50 Ohms
Calculate the turns ratio for the tap point
Calculate the number of turns from the ground end of the coil
One nice person has done the math here - your mileage might vary:
http://www7.brinkster.com/yvesroy/10ft-antenna.asp
an untested Excel spreadsheet is here:
http://scwis.home.comcast.net/Antenacalc.xls
Pictoral info is here:
http://www.part15.us/node/1028
650 KHz is a bit of a pip for an antenna match and at 20 watts you'll want to be aware that your transmitter might be damaged by a mismatch or connection error.
And, of course, not really Part 15 and all that, but I imagine you're aware of that. If you weren't aware of that, it's not.
Experimental broadcasting for a better tomorrow!
Thanks much, the calculations were really what I needed. I'm running a AMT3000 at present with a similar set up with good results. Biggest difference is my coil diameter is 6 inches. While on this subject, can you help with a simple carrier current set up for the 20?
is here:
uses the older "hot side" connections, somewhat dangerous, however.
You can also use the Neutral circuit on AC service and tune it like a long wire, as noted in the links below, which include some additional CC background info.
Installation info from LPB, Inc.
- Ground Suggestions
- Campus AM Antenna Systems
- Carrier Current Broadcasting Theory
- Carrier Current System Design
- Neutral Loading Option
- Carrier Current System Maintenance
Experimental broadcasting for a better tomorrow!