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.
After adding the loading and retuning the radio to 3521, the SWR curve moved down the band and here it is centred on 3521:
A nice application for the SWR indicator and sweep function in the IC706.
Recently I was reminded of a conversation I overheard between two hams discussing which type of beam antenna is best, a full size type or a multiband antenna with traps in the elements to enable it to operate on multiple bands.
One of these fellows was like the experts at the pub, who can advise you on anything from what is wrong with your car to what is wrong with your computer, TV, VCR and your dog.
His opinion was that multiband trapped antennas were nothing more than rotary RF chokes. His logic was that RF chokes have coils in them, so do traps, therefore a trap is an RF choke. His conversation partner agreed with him with a bit of a nervous laugh. This was a seemingly plausible argument but a bit worrying because most people use this type of antenna and they do seem to work.
The fact is that this is a silly conclusion to draw. Every radio has coils inside it to provide essential tuned circuits. That doesn’t mean they are RF chokes, preventing the transmitter signals from emerging on the antenna connector. How could it produce 100 watts at that point if the coils were choking all the RF?
However this kind of statement, if uttered with the right level of assurance and confidence, will feed silly ideas into the heads of less well informed listeners, asking them to suspend disbelief and accept such nonsense as fact.
A little thought about the traps in multiband antennas will reveal them to be specially selected sizes with a specific calculated inductance, to do their job and allow each element to exhibit multiple resonances. They are not RF chokes. RF chokes are coils with enough inductance that they present a very high impedance at the nominated operating frequency. Perhaps our self styled expert thinks that is how traps work. It isn’t.