The AM Forum

Can anyone tell me if this will work?


Each wire section is 12ft in length, each coil (CCCCCC) is 103 turns over 3.5" on a 1" coil form, and both sides are symmetrical.

Key Questions:

does a trapped dipole still exhibit a 75ohm feedpoint impedance?

If not, how do i figure it?

Thanks guys!

What you have described is actually a loaded dipole instead of a trapped dipole.  A trap would be used to isolate sections of the antenna so the dipole would appear resonant on two or more desired bands.  Loaded antennas can work well but in this case you are going to have a lower than 75 ohm feedpoint impedance and a very narrow bandwidth on 80 meters.  Basically your coils on either side are loading coils, not unlike those used in shortened mobile antennas.

Is your basic reason for choosing this antenna to fit it into a small space?  If so you might consider an inverted V type configuration and if you need to bend or "zig zag" the legs some to fit the available space you could do so with generally less loss than you would get using these coils to make up so much of the antenna.  In general for a dipole like this the closer you can come to the standard length (around 66 feet per side for 80 meters, the basic formula is 468/freq. in Mhz.) then the better it will perform.

However, if the antenna you described is all that you can fit then go for it because any piece of wire that accepts power is going to radiate; just at a suboptimal level in compromise cases.  As a novice I worked a couple of stations around 10,000 miles away with 75 watts on CW using a 40 meter dipole attached to the house eaves.  So don't let the antenna constraints keep you from having fun on the air; you don't have to have a killer contest station to enjoy ham radio.

73, Rodger WQ9E

contact the slab bacon on here for a shortened antenna design that really works. needs a tuner but it gets out like gangbusters.