Myths and Legends

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.