Remote Automatic Tuner: with Balun, Unun or Nothing at All?

BL2 Elecraft balun

The multiband wire antenna in my QTH is an end-fed wire thrown up a Maple tree in the backyard. The wire is about sixteen meters long with the shape of a long “S” (with an open hook at the bottom and a closer one at the top). It is complemented by a single set of counterpoise quarter-wave length wires (cut for about mid-band range frequency) for each of the HF bands (80m to 10m). These counterpoise wires make a horizontal “L” elevated about one meter from the ground and running around the fence that encloses the yard. (the 20-meter long wire used for the 80m band is folded at its end back on itself for about five meters). The antenna and the counterpoise set are fed via a short 50 Ohm coaxial (about 30 cm), going through the wall. Inside it connects with theLDG RT-100 tuner, itself connected to the transceiver via a 5-meter coaxial, an Elecraft WII wattmeter and the RC-100 remote controller Bias/T that provides power to the RT-100 via the coaxial.

The short coaxial connecting the RT-100 to the antenna is a compromise for keeping the tuner inside the house (actually, in-between walls) protected from the elements. This tuner was chosen for being both able to operate QRP and QRO. This was important because, although the antenna is mostly powered by an Icom 703+ operating QRP, mainly for SSB and net participation, it is often connected to an ICOM 706MKIIG operating at 50-80 Watts.

The LDG RT-100 is a very reliable remote auto-tuner, initiating a tuning cycle whenever the SWR exceeds a preset limit (usually beyond 2.5:1). However, a push button in the RC-100 Bias/T allows to trigger a new tuning cycle when required.  The Elecraft WII meter is a good visual improvement over the meters in the ICOM radios for determining when minimum SWR has been reached.

As many other users have also found, the LDG RT-100 tends to stop at SWR values close to 2.0:1. This does not seem to significantly affect the output power. Nevertheless, it is puzzling to see it reach lower levels as it tunes, but then settle on a higher value closer to 2.0:1.  One way to influence this is via the use of a balum or an unum.

Here is a table showing the SWR minimum values at which the LDG RT-100 stops cycling for mid-range frequencies in the CW portion of each HF band, when using different balums, unums or nothing at all (minimum values are shown in italics):

Freq. Unun Balun None
10:1 4:1 4:1 1:1
28.0250 1.5:1 1.5:1 1.7:1 1.7:1 1.5:1
24.9100 2.0:1 1.7:1 1.7:1 1.5:1 1.7:1
21.0250 1.7:1 2.0:1 1.5:1 1.7:1 2.0:1
18.0800 2.0:1 2.0:1 1.7:1 1.7:1 2.0:1
14.0250 2.5:1 2.5:1 2.0:1 2.5:1 2.5:1
10.1150 2.5:1 2.5:1 2.5:1 2.0:1 2.5:1
7.0250 1.7:1 2.0:1 1.4:1 2.0:1 2.0:1
3.5250 2.0:1 1.7:1 1.5:1 1.7:1 1.7:1

Although the differences do not appear to be too significant, the use of a 4:1 balun seems to offer to the RT-100 the best matching opportunities for the antenna described above. Furthermore, the possibility of using a Elecraft BL-2 wideband balun switchable between 4:1 and 1:1 (see image at the top) seems to offer the best possible arrangement (even if the antenna can hardly be described as “balanced”).