Tag Archives: helical

More about the Alligator Hat

The June 2009 edition of AR carries a small article I wrote about lowering the resonant frequency of a HF helical antenna to allow me to use it on a lower frequency than its design centre frequency. My 80m helical, for example, is resonant at 3585 and provides the lowest SWR at that frequency.  At the CW end of the band, say 3520, the same antenna has a SWR of over 3:1, sufficient for the IC706 to cut back its output power to less than 10w.  To operate on the CW end of the band with this antenna I therefore need to deal with the mismatch using an ATU, or change the resonant frequency of the antenna.

The method I used was to load the helical with a capacity hat formed by a short wire, actually an alligator lead I had in my field day accessories box.  I was operating from the car and was nowhere near home at the time.

Here is the SWR curve from the front panel of the IC706, with the radio tuned to 3585 kHz.  The SWR bar-graph is small but the general shape of the SWR curve can be appreciated.  The bars represent the SWR at frequencies below and above the centre frequency indicated by the main frequency dial.  The small dot below the bar in the centre of the graph reminds you that is the measurement corresponding to the dial frequency.  The increment per measurement is 10 KHz, as set using key M2.  The white rectangle drawn around the SWR bar graph was added to the photo by me.

SWR centred on 3585
SWR centred on 3585

After adding the loading and retuning the radio to 3521, the SWR curve moved down the band and here it is centred on 3521:

SWR of loaded helical, centred on 3521
SWR of loaded helical, centred on 3521

A nice application for the SWR indicator and sweep function in the IC706.