I decided to add an extra 120′ (36m) tower to have a better signal in contesting. My signal from the current 100′ (30m) is not competitive on 20m and 40m so I decided to upgrade a 2 el 40m beam to 3 elements and 3 el 20m beam to 4 elements.
I decided on a 17-4 4 band beam from SP7GXP. Due to tariffs and UPS fire it came late. When it was ready to install, my installer got it to 110ft, could not do any better, and looks for help to put it on top. All while the beam is grinding in wind.
I planned to use the tower for strongly bent 4 square. Since the beam is not installed, I temporarily set up only one element towards NE with only 5 18m radials. The vertical resonated at 1.817 with a resistance of 57; vertical with many radials have resistance of about 35 Ohms. My expectations were low.
I started testing using RBN and comparing to my current 100ft shunt fed tower with 36 30m radials. To my surprise, nearly all reports (that seemed to be from the NorthEast) favored the new antenna by an average of 4 db. However, some stations were weaker: from AR, TX and Cayman Islands. Apparently, the tower with the unfinished beam is acting as a reflector.
I added 3 more radials and the impedance dropped to 47 Oms while resonance increased by 10 KHz. Now reports seem to be even 1-3 db better. Up to 9 db improvement over the shunt tower.
Assuming identical radials, the impedance at resonance of a vertical is approximately:
R=a + b/n, where n is the number of radials. Fitting two data points: 57 Ohm with 5 radials and 47 Ohm with 8 radials,
the formula is
R = 30 + 133/n.
30 Ohms is a bit low but nor far of the ballpark. With 16 radials the impedance would be 39 Ohm.
Assuming that the term 133/n denotes ohmic losses, the efficiency is 30/(30+133/n). According to the formula, the efficiency with 8 radials is 30/47 is 64% or -2 db from the perfect one. With 16 radials the efficiency would be 78% or -1 db down. My formulas is likely an approximation since longer radials are required for higher efficiency. Anyway, with just 8 radials I have a signal up to 9 db better towards Europe that shunt-fed tower.
Good results with the new vertical make me suspicious of the shunt fed tower. It is matched by a 12-gauge wire (2mm) and matched by capacitors. Rather flimsy construction since the vertical wire is supported only on top and at the bottom.
When I transmit, the amplifier turns itself with high SWR message initial at 800W then taking up to 1200W. All while the SWR meter shows SWR < 1.2. In contrast, the new vertical can take up to 1450W. While scanning, the SWR fluctuates like with intermittent connection. However, I was not able to find any missing weak point in the feed. Since beams are part of the tower, there is a possibility that coaxes feeding the beam are not well grounded or something else on top of the makes bad contact and is sparking, e.g., guy attachments.
My big worry now is that with the beam on top the tower will no longer be a reflector! On the other hand, the shunt fed tower may be weak only because the new one blocks signal from the NE.