[Elecraft] Re: Ferrite, toroids and the KX1

[Ron D'Eau Claire]

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: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of kk7ss at verizon.net
Sent: Friday, September 19, 2008 2:10 PM
To: elecraft at mailman.qth.net
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
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