[TowerTalk] Coax loss. Has anybody measured it ?

[Donald Chester]

When I first put up my 160m vertical, I used a 140'  run of RG-214, mil-spec silver plated doubly shielded. Terminating the far end with a 50-ohm dummy load, I used my  so-called RF watt-meter and then verified the readings with a thermocouple RF ammeter. The fresh coax showed around 93% efficiency; 100 watts at the transmitter end resulted in 93 watts at the far end. I thought the efficiency should  have been better than that on 160m, and the reading was only slightly worse on 80m.

The coax ran on top of the ground for several months, and then was buried a couple of inches below the surface to protect it from lawnmowers and surface traffic. A couple of years later, I re-checked efficiency, and it had dropped to about 80%. I examined the coax and found that some kind of critter had chewed holes in the jacket, exposing the braid.

I replaced that coax  with a fresh batch of RG-213, purportedly rated for direct burial. Interestingly, the efficiency of the fresh RG-213 was nearly identical to that of the fresh RG-214. I buried the 214 in the trench left behind from the 213. After a couple of years, I noticed the same drop-off in efficiency, except it was a  little worse, slightly less than 80%.  Upon inspection, I could find no evidence of chewing on the jacket. Apparently, the "direct-burial" stuff still managed to absorb moisture.

I set a line of 8' high poles between shack and tower. At first I used a run of fresh RG-213 on the poles, which once again showed about 93% efficiency, but decided to go a different route, using open wire feed. I built the OWL with two pieces of #8 copperweld, spaced 2 1/2" apart, which yields 438 ohms surge impedance according to the formula
(I used the stiff, cantankerous copperweld because I had a couple thousand feet on hand). The OWL is mounted on metal cross arms at the top of the poles, supported by 2 1/2" high ceramic stand-off insulators. I ran an efficiency test using a thermocouple ammeter, working into a 450-ohm dummy load at the far end. The efficiency calculated to 98%!  I reworked the matching networks to accommodate the balanced OWL so that it operates essentially with no standing waves. Comparing RF current readings with a thermocouple meter in series with the base of the 160m vertical, using the coax feed system and then the OWL, I noted a substantially more base current with the OWL, with identical DC inputs to the final stage of the transmitter with each system.

I now use that elevated run of coax as portion of the long feedline to my beverage receiving antenna located at the far end of the antenna field beyond the radial system, and OWL feeds all my transmitting antennas for 160m, 80m and 40m. Except for the receiving beverage, I no longer have a sprig of coax anywhere in my entire antenna system. 

The 137' long dipole is mounted at the 119' level on my tower plus runs of Phillystran and a couple of wooden poles some distance away to support the ends, with tuned OWL feeders (meaning they operate with standing waves) from the matching unit at the base, up the tower up to the dipole. The OWL runs up through the interior of the tower, spaced at the geometric centre of the triangle, held in place with plexiglass spacers every 10'. The close proximity of the OWL to the tower along its entire length results in close coupling between the tower and the dipole, which adds substantial top loading to the vertical; the base impedance measures in hundreds of ohms + substantial "j" factor, as opposed to the theoretical 36 + j0 ohms for a quarter wave vertical. 

I doubt  that the increase in efficiency of OWL over coax makes any perceptible difference in signal strength at the other end of the QSO, but I still find satisfaction in knowing I am not wasting that additional 5% of my RF power in the transmission line, given that there must already be measurable losses in the transmitter tank circuit as well as in the ATU at the base of the tower. But the main advantage is knowing I won't have to replace the OWL every few years after transmission line losses gradually creep up.


Don k4kyv