[TowerTalk] Inverted Vees

[Donald Chester]

Grant Saviers grants2 at pacbell.net Mon Jun 15 14:09:50 EDT 2020 wrote:
> It's a tough game to achieve low angles from horizontal antennas on 80m 
> until over 1/2 wl high.

My 80m dipole is attached to the tower at the 120' level, with the ends drooping down to 100'.  With that small amount of slope I consider it closer to a true horizontal dipole than an inverted vee.   The feedline is open wire line, two #10 conductors spaced 2" apart, running up through the interior of the tower, spaced at the geometric centre of the cross-section triangle of the Rohn 25G.

The 127' tower is insulated at the base, with 120 uniformly  spaced buried radials, each 133' 4" long.  The base-insulated tower is used as a quarter-wave vertical tee for 160, with the dipole serving as the horizontal portion of the tee.  Even though neither the dipole nor the feeders are electrically in contact with the tower at any point, the  close proximity of the feedline to the tower over its entire length causes the dipole to be closely enough coupled to the tower to add substantial top loading; as a result, the base impedance of the tower on 160 is in the range of hundreds of ohms with significant reactive component, rather than ~38-40Ω + j0 as it would be expected with just the tower with no dipole attached.

A few  years ago I tried loading the tower as a half-wave vertical on 75.  As expected, the performance was mediocre at best at less than 600-800 miles, but I had interesting results working DX.  I contacted the same French station on 75m AM phone several times, as well as stations in the pacific northwest, and asked for signal comparisons between the half-wave vertical and the (nearly)  half-wave high horizontal dipole.  In every case, at both distant locations, the reports were that  the signal strength peaked about the same with both antennas, but with the vertical tee, the fades were much deeper.  The signal was decidedly more solid with the dipole.

I had expected the half-wave vertical tee with 120 half-wave radials on 80m to be a killer antenna out beyond 2000 miles or so.  I was a little disappointed that it didn't perform quite so well as the dipole. I never tried the comparison at KH6, VK/ZL or Asia; maybe that's where the half-wave vertical would have dominated. 

I rarely work DX, so since then I have stuck with using the dipole on 75-80m. The 80m dipole will load on 160 as a quarter-wave dipole (using the appropriate ATU).  Normally on 160 the vertical is about 10 dB stronger at points beyond a couple hundred miles, but at locations 50-100 miles away, in the evening the 80m dipole may be as much as 30 dB stronger than the vertical tee.

I also use the dipole as a double-zepp aka two-halfwaves in phase on 40m with good results.

Don k4kyv