Auto-tuner Friendly.
In planning a return to South Africa, with also include a one-week side jaunt to St. Helena Island, I was wanting more bands to be available than during either of the prior two trips. Nothing less than 10m through 40m, WARC bands included, would serve. Like so on a single homebrew antenna.
With the above as my goal, an auto-tuner was clearly called for. And while my RGO One rig did come with an ATU installed, that would still leave high SWR in the feedline. A major bane to weak-signal reception. Better to have a stand-alone ATU at the far end.
And so I purchased an LDG RT-100. And rather than rely upon its advertised SWR range, I note instead the range of its internal components.
Below are presented the L and C ranges for four common auto-tuners, two built into their rigs, two external.
| μHmin | μHmax | pFmin | pFmax | |
|---|---|---|---|---|
| Mission RGO One | 0.05 | 8.42 | 10 | 2531 |
| Elecraft K3 | 0.05 | 17.34 | 12 | 2627 |
| LDG RT-100 | 0.11 | 10.17 | 10 | 1270 |
| ATU-100 | 0.05 | 8.42 | 10 | 1869 |
Thereby are my design constraints now set. The antenna must be tunable via a low-pass L network needing values for C and L of no greater magnitude than in the table above.
For the trip, I had also purchased a Spiderbeam mast. Those being prone to collapse under certain conditions, I choose to locate the auto-tuner near ground level. From it to the radiating antenna, I run 300-Ohm window line instead, with RG8X between the auto-tuner and rig.
The window line is simply feedline, not a stub or anything else.
This is the length which AN-SOF 9.5 predicted would prove auto-tunable. And so it did prove. Happily it is also not over-long. On prior trips I had strung doublets of either 9.312m and 11.348m, these both with 9:1 baluns at center. Both of which required a second manual tuner ahead of the RGO One’s built-in auto-tuner for certain bands. So this antenna is both better and shorter.
Here are plots from AN-SOF 9.5 as a horizontal. Not bad at all (for a random) on the upper HF bands.
From 20m through 60, the patterns are just only so-so.
Deployed from a Spiderbeam mast, horizontality is hard to achieve. Here are the legs drooped to 30 degrees.
Plots from AN-SOF 9.5.
From 20m through 60, the patterns are just only so-so.
While I hadn’t yet boarded the plane for Cape Town, I deemed this design sufficiently proven by virtue of testing in my side-yard at my KY8D home QTH. The original model was as a horizontal, 10m above perfect ground. The testing was instead as a shallow inverted V as per the depiction above. I later re-modeled it, this time drooping at 30 degrees, above dry, sandy, coastal ground.
The LDG RT-100 proved able to tune the antenna from 10m through 60m. This despite the model predicting 60m as out-of-range. QSOs were got only on 10m, 20m, and 30m, before I took the doublet down to try various alternate lengths (none of which much impressed). The 20m QSO was with 5R8IC in Madagascar, which surely makes it a keeper.
Now back from our trip, I’m pleased to report that the antenna proved fully as versatile as was my hope. In the photo below, see it deployed on the deck of Chiduku House in Simon’s Town, South Africa, a few streets up-slope from the historic naval base. Note the 300 Ohm ladder line leading down. The LDG RT-100 is not in view for hanging suspended just underneath the deck, tethered in place by Dacon kite line.
During our three months in South Africa we made a few side trips. During a few two- and three-night stays, I deployed various other, far more temporary antennas. This one, however, was my mainstay, remaining in place at Chiduku House all the while. Mostly I left the ends tethered genereally to NE/SW. This in avoidance of the metal roof. At one point I did try out reorienting it 90 degrees, but restored as shown in the photo after a week. For a full accounting, visit my QRZ page for ZS1KYD: LINK
Latin for “Let the moocher beware”. I composed this for me, and employed it with glee. I share it here with you for free, but offered without even so much as the smell of a dead warranty (or wheriot, as the case may be). Whatsoever disasters of cosmic proportion as may result from the use of this information, any and all culpability rests wholly and solely with the user.
My contact email and postal address are on QRZ.com for call signs KY8D and ZS1KYD
