Antenna guru I'm not, but this antenna grasshopper has a little information that may help.
The "loading coil" is used to compensate for reactance in the antenna. A grounded, end-fed < 1/4 long will present capacitive reactance. You simply need enough inductance to bring it to resonance. Normally, the ATU will provide that unless, as you noted, it can't provide enough. The only purpose for achieving "resonance" is for efficient power transfer. For an AC circuit (RF is just high-frequency AC) to transfer power efficiently, the current and voltage must be in phase: maximum voltage and maximum current must occur at the same time. That won't happen if reactance is present. So the first step is to achieve resonance. That leaves only the resistive part of the impedance presented to the rig. That's the part that consumes the power. Part of it is real "resistance" - the resistance of the conductors in the antenna. The rest is "radiation resistance" - the part that becomes electromagnetic (radio) waves. The value of that resistive component varies widely. At the center of a half wave (dipole) radiator, it's about 75 ohms in "free space". At the real heights above earth us Hams are usually forced to use it's usually closer to 50 ohms. Either value is an excellent match to the popular 50-ohm coaxial lines. A "Marconi" antenna (grounded 1/4 wave radiator) will show half that value or about 37 ohms. But, as you make the radiator shorter than 1/4 wave, the resistive value plummets. At 1/8 wave the value drops to about 7 ohms. Your 21 foot antenna will be about 1/12 wavelength long on 80 meters. Its resistive impedance will be about 3 ohms. Your ATU will need to provide an impedance transformation that matches the roughly 3 ohms to 50 ohms. You'll get a 1:1 match if it can do that. Adding or removing turns from the loading coil won't affect that resistive value (other than to change the added conductor resistance), but it can reduce the amount of inductance the ATU must provide to resonate the system, and that may allow it to find a match. The fact that the resistive part of the impedance drops so quickly to very low values is why short antennas are inefficient at best. The conductor resistance of the coils and antenna quickly become equal to or larger than the radiation resistance. Since the RF current flows only along the very surface of a conductor, its RF resistance is much, much higher than its d-c resistance. It's not unusual for a short, loaded Marconi antenna (such as a mobile whip with loading coil) to burn, literally, most of the RF power applied in the conductor resistance. If we ever get really cheap room-temperature superconductors, we'll be able to put up a really efficient short whip antenna on the low bands. Until then we groan and live with it, getting all the physical length we can to raise the radiation resistance and cut our losses. Ron AC7AC -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Friday, September 19, 2008 2:10 PM To: [email protected] Subject: [Elecraft] Re: Ferrite, toroids and the KX1 Just to confuse the subject... ;^} I am running a KX1 with an antenna lenght of 21' and a 'ground' length of 21'. The KXAT can match these to < 1.2:1 on 40/30/20 with little effort. I'm taking the approach of trying to use a ferrite toroid based inductive load in series with the antennna wire. By trail and error, I have constructed a F50 toroid (mix unknown but the whole toroid is plain black) that measures out at 40uH and ~800X at 3.55 Mhz using an MFJ-269 Pro.... This weekend I intend to try inserting this between the KX1 BNC socket and the antenna wire and, if I have the time, in the middle, to see if the KXAT can give me a match... I don't think end loading will be practical... Any input from the antenna guru's out there would be welcomed... Dave KK7SS _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
