[Elecraft] MFJ 20-meter whip on 17 and 15 meters with the KX2 (or KX3)

[Wayne Burdick]

Hi all,

Today I was operating pedestrian mobile in the ARRL DX contest with just the MFJ1820T 20 meter whip. This 4 foot-long whip is very narrow banded, which isn't a problem over the full 20 meter band as long as you have an ATU available - the KXAT2 in the case of today's KX2 excursion. (Note: You also need a ground counterpoise. 13 feet is about the right length.)

When trying to use this whip on other bands, in a pinch, it can be tough on the ATU, given the sharp resonance of the antenna. But I realized I hadn't tried shortening the telescoping part when operating on frequencies above the target band.

It turns out that, at least in my case, the ATU could match to 1:1 on 17 meters by collapsing 2 lower sections. On 15 meters, I could get a 1:1 match by collapsing 5 lower sections. 

While this is certainly a compromise in terms of radiation efficiency, it does make the transmitter happy, allowing the use of full power. I didn't do any far-field tests to compare the shortened whip at 1:1 to the full length of the whip at much higher SWR, but for receive purposes, it was about a wash. 

As a bonus, there's less metal waving around in the air. This can be handy when you're walking, especially in a residential area with a lot of "Neighborhood Watch" signs proudly displayed :)

73,
Wayne
N6KR