A Communicator Reprise: June 2013
The RF Demons Are Finally Exorcised
I purchased the Carolina Windom 80 antenna from Radio Works with the idea of using it on 80 and 40 m to supplement my SteppIR vertical, which covers the 6m-20 bands. Being 133 ft. long, it is a full ½ wave for 80 m and advertised as usable on the higher bands. One of its distinguishing features is a “vertical radiator”, a 22 ft. length of 50 ohm coax connected off-centre to the horizontal radiator by way of a 4:1 balun. At the lower end of the vertical radiator is a choke balun (or “isolator”), there for the purpose of discouraging radiation from the remainder of the feedline connected to the transmitter. Presence of the vertical radiator, it is claimed, changes the radiation pattern to give a low angle component, good for DX. When I searched the web for comments on this antenna, virtually all indicated that the owners were happy with its performance and would recommend it to others. I had some challenging issues when I put the Carolina Windom up at my QTH.
I have a number of tall trees on my property, but only one not-very-tall tree suitable for anchoring one end of the wire at the front of the lot. At the rear of the lot are several taller trees, none of which is ideally situated to anchor the opposite end of the wire. Nevertheless, I used SARC’s air cannon to fire a line through one of these trees about 50 ft up. When we raised the antenna, both ends of the wire came close to branches, which raised the concern that they might affect the performance. In addition, another tree near the middle of the lot had drooping branches that came very close to, and sometimes touched, the suspended wire.
I then proceeded to make some measurements using an MFJ-269 analyzer. The MFJ reads not only SWR but also the components of Z (impedance), which are Rs (radiation resistance) and Xs (reactance). When I pulled the antenna up to full height into the trees, the measured SWR was above the range of the MFJ (>31) over most bands. Rs (which I expected to be around 50 ohms) was zero nearly everywhere. I then repeated the readings with a Comet CAA-500 analyzer with the same result: every SWR reading on every band was off-scale. Clearly, something was wrong. When I transmitted and put power into the antenna, sometimes I could tune it with the radio’s tuner and other times I could not. The antenna was then taken down and all parts checked to see if there was a faulty component. Both baluns tested OK, the coax connected to a 50 ohm dummy load appeared good, and all solder joints seemed to be sound. I even substituted known good baluns for the ones provided but nothing changed for the better.
I then commenced my email correspondence with Jim, the Radio Works guy to explain the symptoms. He was puzzled by the results and after several emails back and forth, suggested I return the antenna to them for checking, which I did. I received it back within a few weeks after Radio Works had replaced the vertical radiator because of a suspected intermittent connector. The winter months went by before I put the antenna up again, and began testing to see the results. In the meantime, as related in the previous posting, I had acquired an AIM 4170 antenna analyzer and used the time to get familiar with its many features. I ran the AIM through some tests with my other (resonant) antenna and confirmed that the MFJ and the AIM gave almost identical results for SWR and the components of impedance (Xs and Rs).
Now for the Windom.
Figure 1 shows the initial setup. The antenna was about 35 ft. off the ground at one end and 50 ft at the other. 60 ft of 50 ohm RG213 transmission line connected the line isolator to a surge protector and station ground at the house entrance panel, then to an external swr meter and the transmitter.
Figure 2 shows the results of SWR measurements across all bands from 3 to 30 MHz (note: the amateur bands are highlighted). The SWR trace displayed dips at certain frequencies but mostly not where they should be.
Figure 3 shows the results of several re-scans for the 80 m band, and
Figure 4 for 20m – every re-scan gave a different pattern. When I tried putting power into the antenna from the transmitter, it usually would not take power or, if it did, it was only momentary while the internal tuner kept searching for a match on each key-down. The results were no better on other bands, and rainfall also made things even worse.
Figure 5 shows the result for the 80m band under both conditions. This was possibly an important clue.
Since I had thought that the interfering branches might be affecting resonance (as they blew in the breeze), my next move was to have the trees pruned to eliminate the offending branches. At the same time, I got the tree guy to install a rope and pulley system (made from bicycle inner tubes, shackle and pulley) around the trunk of two select trees about 40 ft. up so I could raise and lower the antenna at will. The wire, 4:1 balun and coax were then raised using the pulley. The spacing between the trees did not permit the wire to extend full length, so I put a drop leg on the short end of the wire and pulled it taut. The coax connected to the “middle” tree followed the trunk vertically down to ground level, snaked in a circuitous route across the garden and up into the second floor shack. This required double the length of coax compared with the previous arrangement. As before, the coax outer conductor was grounded at the entrance panel surge protector but this time I added a choke balun at the entrance panel by coiling up the extra 15 ft of coax, which I connected to the transmitter by a 15 ft jumper.
Figure 6 shows the new configuration.
Figure 7 shows the 3-30 MHz scan, and
Figure 8 the 80 m scan after these changes were made.
The SWR across all frequencies and especially within the amateur bands was much lower and mostly below 3; successive re-scans of SWR, Xs and Rs gave virtually identical results. The SWR was now acceptable on most of the 80 m band and the match to the transmitter was stable. Other bands (except 10 m) showed SWR 2-4, higher than I would prefer, but within the range of most internal radio tuners and a definite improvement over the initial results. Also comforting was the fact that my external SWR meter connected to the transmitter (under power) displayed the same SWR as did the AIM.
To effect this dramatic improvement, I had made a number of changes all at the same time. So I started undoing the changes one-by-one to determine which was the critical one. I had previously confirmed (to my surprise) that the pruning of tree branches had no discernible effect. Lowering the centre of the antenna to its previous height did not do it. Removing the ground at the entrance panel had no effect, nor did uncoiling the choke balun. The only way I could recreate the original problem was when the antenna was lowered to its original height, the feedline was shortened from 120 ft. to 60 ft. and the coax was allowed to make a drooping loop from the antenna to the shack rather than falling vertically from the antenna and taking a circuitous route to the shack. Now this does not provide a technical explanation of the problem; it only explains what had to be done to fix it. I do believe that I had RF on the feedline initially. What have I learned from this experience?
- Get the antenna as high as possible
- Make sure the feedline drops down vertically to the ground
- Change the length of the feedline if there appears to be a serious mismatch
There is one further puzzling footnote to this situation: Even after the aforementioned improvements, the MFJ meter consistently reads higher – by a large margin – than both the AIM and the external SWR meter.
For example, on 80 m, an SWR of 2 on the AIM reads 8-15 on the MFJ. Xs and Rs also disagree with the AIM, with Rs equal or close to zero on the MFJ most of the time. I am interested in thoughts of our more knowledgeable members regarding all of the foregoing. However, at least now I have an antenna I can use.
~ John Brodie VA7XB