Modification of the Cushcraft 416TB (2x8 elements) 70cm antenna
(add switchable circular polarization)
Cushcraft provided with the 416TB antenna a polarization 'phasing harnesss' that allows either fixed LHCP or RHCP.
For years I used RHCP and was fully satified with the performance (AO51, VO52, FO29, Cute-II, ...)
Click here for the original Cushcraft 416TB Manual (pdf).
Below a picture of the harness as I had it mounted (fixed RHCP) for years:
With AO51's 435.150 MHz downlink I had an issue as that applies LHCP. I could clearly notice the signal strength difference between 435.300 (RHCP, strong) and 435.150 (LHCP, barely audible). Then when AO51 started to apply repeater modes on 435.150 as well, which I now was starting to miss out on, I wanted to be able to switch between LHCP and RHCP.
As I had only one relay available (these projects alway start on Sundays when shops are closed), the option of HOR/VER/LHCP/RHCP was dropped and only switching between LHCP/RHCP remained.
From various sources on the internet and an article of Domenico Marini, I8CVS, ("Switching four Polarizations on a 70cm Crossed Yagi", AMSAT Journal May/June 2007) I derived the set-up needed for the Cushcraft 416TB, which has both antennas at 1/4 lambda distance separated from eachother (A - B in the diagram below):
The original Cushcraft harness basically consisted of Z1 and Z2 only, and RCHP vs LHCP was selected by swapping the conductor & shielding at the feedpoint of the front dipole.
In the new above set-up, LHCP is obtained when the relay is excited (signal travels through Y2), else the resulting polarization is RHCP.
Z1 and Z2 form the impedance transformers, which transform the 50 ohm of each dipol into 100 ohm, which are then connected in parallel resulting in 50 ohm. Y2 is the phase shifter: if enabled the signal from/to the front dipol is shifted 180 degrees (1.2 wave).
For Z1 and Z2 I used RG59B/U (75 ohm): V=0.66: ((300/435)*0.66)/4= 11.4cm.
X1 and X2 just need to be identical in length, I have used 40 cm Aircell (50 ohm).
u1 and u2 need to be identical in length, u2 is a small piece of copperwire between the relay pins (~4mm).
Y1 needs to be 1/2 wave longer as u2, I used Aircell (50ohm): V=0.83: (((300/435)*0.83)/2)+4mm= 29.0cm
U1 is created by soldering the conductors of the two coax cables directly together and putting a closed copper shielding around that (by soldering the shieldings/braids of the two coax cables together and around the conductor) to form again a shielding. Refer to point 'u1' in the picture of the box below.
Domenico Marini advices in his article to use proper coax relay and in general describes a more perfect system, including HOR/VER/LHCP/RHCP options. I have chosen a much cheaper and simpler implementation, which is most likely also less accurate. So for those that want to built this system as well, I recomment them to read the article of Domenico first and decide themselves how to built. Do not only refer to my implementation described here.
I have used a simple SDS S2-12V relay, which is expected to work up to 450 MHz. Key is to keep all wires as short as possible. All coax braids (shieldings) have to be soldered together via the shortest possible way. See the yellow circle in the picture of the box.
Parallel to the relay I have mounted an LED, which makes it easy to be able to see without a voltage meter (eg outside standing at the roof) if the 12V is present in the box or not.
Again: using a real coax relay would have been much better. Using a waterproof metal box instead of my plastic one would have been better as well, but I didn't have one and it would have made the construction much heavier.
But as I am not aiming for an EME station I decided to give this implementation a try.
First I stripped the antenna and removed the original phasing harness:
The baluns are carefully removed and cleaned as I needed them also in the new set-up:
Next the actual switch was made and placed in a 220V outdoor box (waterproof) and mounted in the tower:
Just comparing the 435.150 and 435.300 downlinks of AO51 I concluded the system works:
- In the past I could not hear the 435.150 downlink. Now I can; by selecting the correct polarization.
- Signal strength of the 435.300 downlink has not decreased compared to the past (the fixed Cushcraft harness).
With a few pieces of coax, a waterproof housing and a relay a Cushcraft 416TB can be upgraded from fixed Circular Polarization (CP) to (remote) switchable Left or Right Hand Circular Polarization (LHCP or RHCP).
Implementation has been done using spare parts as I needed LHCP/RHCP to become available quickly. However, a next time, instead of a normal relay, I will use a coax relay as it is expected to give better mechanical stability and improved RF performance.
Henk Hamoen, PA3GUO