[Elecraft] Vertical with short radials (was: Home made Sigma-GT5 & KRC2 or SGC?)

[Vic Rosenthal]

Mike McCoy wrote:

> In my particular situation I am becoming a big fan of vertical dipoles
> because they are small footprint, don't require radials yet still provide a
> low takeoff.  

I just built a new 40-meter vertical.  Following the advice of G6XN (Les 
Moxon: HF Antennas for All Locations), I made a groundplane with 
shortened, loaded radials.  I was able to put up a 33' (10m) vertical 
radiator and I used four 8' (2.5m) radials made of aluminum tubing.  The 
radials are fed through a coil of about 8 turns of no. 12 wire, 3" (7.6 
cm) in diameter, which I adjusted for resonance.  By adjusting the coil 
and the the length of the radiator, I got an SWR of about 1.3:1 at 
resonance (7.03 MHz).  The antenna was mounted about 10' (2.5 m) from 
the ground on a steel mast (the radials are isolated from the mast) next 
to a 1-story building, and fed through a bead balun.

According to Moxon, and EZNEC agrees (taking into account the coil 
resistance), there's no significant gain difference between this radial 
system and the usual 1/4 wl resonant radials, and there are significant 
advantages in terms of current balance (and therefore avoidance of 
wasteful radiation straight up) over the usual system.

Unless a vertical dipole can be built full-sized (a half wave vertical 
dipole is hard to build for 40 meters unless you have some very serious 
trees available), it will have to be shortened by some sort of loading. 
   This lowers the radiation resistance, which usually means lower 
efficiency.  In the case of the antenna I'm describing, since the 
radials are self-supporting and part of the antenna, it's no harder to 
install than an R7 or similar 'half-wave' antenna with vestigial radials.

-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco