[Elecraft] trap dipole?

[Stuart Rohre]

Jeff,
You do not have measurable loss using your receiver with CATV line in place
of 50 ohm coax.  TV line is 75 ohms, vs. 50 for the coax.  The difference is
1.5:1 SWR, and no apparent loss would be hearable on receive, nor noticeable
on transmit.

There will be some mismatch loss but with a transmatch, you can match the
whole system to work near 100 per cent efficiency.

Adding even a commercial trap dipole, you will introduce MORE losses due to
reactance and resistive losses in the traps.

The trapped dipole will be more difficult to tune if the sides are not
symmetric, but hams' dipoles that do not run symmetrically are tuned every
day with adequate results.

If you are getting RF back at the shack, look for other causative
mechanisms.  It may be the close coupling of the low part of the dipole.  It
more likely is that on 40m you have a quarter wave of TV coax, or odd
multiple of quarter wave such that RF easily flows over the outside of the
shield, from the dipole feedpoint, and back to the shack area.  Add some
length to see if that is cured.

With a Handbook and some wire and pvc from Home Depot, any ham can make
their own trap dipole, (Or use coax traps) that equals or exceeds the
commercial offerings.  Coax traps have the advantage of having the
capacitance derived from the per foot capacity of the coax cable wound as a
coil.  See the Antenna Book, ARRL, late Handbooks, or other ham Antenna
books such as Bill Orr's W6SAI, Antenna Book.

Why limit yourself to a commercial antenna?  Part of ham radio is learning
new things, and antenna building is one of the easiest for hams to do today
themselves.  If you must go commercial, look at the Force 12 Antennas
vertical dipoles as very efficient, compact and low profile solutions.
However, I would still use a dipole also for those close in contacts, and in
state emergency HF work.
73, Stuart K5KVH