Setback Antenna

The KR8L Setback Antenna

I built this antenna a while ago, and have made quite a few contacts at 2.5 and 5 watts (CW, SSB, and PSK-31) with the rig and antenna sitting on my coffee table in the living room. I finally decided that although this is a pretty good portable and "motel room" antenna, I wasn't happy with it mechanically for Pedestrian Mobile work, so I sat it aside for a while. The following discussion is taken from an E-mail that I sent to a couple of people who expressed interest in the antenna following a QSO. Inspiration for the design came from the Hustler mobile whip (I had just returned from two weeks on the road using a Hustler 22-inch mast on my pickup truck, running 5 watts with my FT-817). A comment by someone on the Yahoo HFPack Group regarding using a 15m resonator on 20m with a longer stinger (as is sometimes done by the maritime mobile crowd) inspired me to minimize the size of the loading coil. Budd, W3FF provided the idea for the Radio Shack six-foot telescoping whip.

What follows is a description of a little thing I have been calling the KR8L "Setback" Antenna (because it attaches directly to the SO-239 on the back of the radio set). I have been meaning to do a detailed write-up on this thing, maybe with some diagrams. Since I may never get to it, here is a quick explanation. By the way, this antenna tunes from about 9.8 to well over 50 MHz.

I'll give you the basic concepts and dimensions here, and the pictures at the bottom of this page should be helpful in following the discussion. The way I put it together sort of developed as it went along.

The lower part of the mast is 1/2 inch PVC pipe, 30 inches long (length is not critical on this part, go longer if desired). I think mine is Schedule 20, but I'm not sure. I screwed a PL-259 into a right angle adapter, then fitted the PL-259 into one end of the pipe. Some PL-259s will slip into the pipe and have to be glued in place, others are too large, but will fit if the pipe is carefully heated over an electric stove (the pipe will scorch quickly over a flame). I used the larger version, and it makes a very tight, stable fit once the pipe shrinks around it. Of course, you need a conductor inside the pipe! I used a scrap piece of RG-11 (RG-8 would work, too). I am using the outer braid for the antenna conductor, NOT the center conductor. The reason for this was to try to emulate a hustler mobile whip, which has a fairly large diameter lower mast. You could try a simple wire, and I am sure it would work. My suspicion is that the larger diameter conductor makes the antenna less narrow banded, but I don't know for sure. (Mine seems to be fairly broadbanded.) You need to make a transition piece (short insulated wire) so that the RG-11 braid is electrically connected to the PL-259 center pin, NOT the outer body. The piece of RG-11 is about 22 or 23 inches long (similar to Hustler's compact 22 inch mobile mast). I removed the outer jacket from the RG-11 since it isn't needed and just adds weight and (probably) dielectric loss.

At the top of my pipe I drilled a hole about 5.5 inches from the top. A piece of solid copper wire is soldered to the RG-11 braid and comes through this hole. (Plan ahead for this before fitting the PL-259 and RG-8 into the pipe.) Another hole is drilled about 2 inches from the top. Between these two holes I wound a coil using #16 enameled wire. This makes 18 turns per inch, so I have about 56 turns. I put taps on the coil at about 9, 18, 27, and 36 turns from the top. The copper wire from the braid connects to the bottom of the coil, and also to a short jumper wire with an alligator clip on it. This clip connects to one of the taps to tune the antenna (or is left unconnected to use the full coil).

Now for the top. I used another PL-259 knurled ring. I took a Radio Shack 6 foot telescoping whip (don't use a shorter one, you need almost the full six feet), and soldered a short length of solid wire to it. Remember that hole drilled 2 inches from the top? This wire will stick through that hole and connect to the top of the coil. I put the whip through the PL-259 knurled ring (just the ring, not the PL-259 itself) and held it in place while I filled the gap with hot-melt glue. That was tricky! Once it was set, I heated the PVC pipe and pushed the ring into the top. (When warm the pipe is very pliable. Don't use too much force or you will distort it.)

One last thing: the secret to this antenna is a quarter-wavelength counterpoise connected to the rig's ground connector. I have a counterpoise that I use with my random wire tuner that has connectors in line so that I can adjust it to be a quarter wavelength on 40, 30, 20, 17, 15, 12, and 10 meters. This adjustment is very sensitive. Try changing bands from, say, 20m to 17m using this antenna and you will discover that although the difference in length of the counterpoise is only a couple of feet, you must use the proper length of counterpoise or the SWR will go sky high. Of course, the counterpoise must be insulated from ground. To calculate the length I used feet = 246/f instead of 234/f, since the latter formula reduces the length of an antenna by 5% for "end effects." I don't know if this is appropriate or not, but mine works OK.

Here is how mine works out. T# refers to the taps; T0 is alligator clip not connected to a tap (full length of coil is used), T1 is first tap from bottom, T2 is next up, etc. T5 means coil is bypassed completely. In general, use the least inductance you can and the longest whip, as this gives the lowest SWR. I found the antenna to run about 35 ohms, with which the FT-817 is quite happy. If you make the lower section just a few inches longer, then you can bypass the coil on 10m and extend the whip completely.

Band (m) Tap Whip (in.) Inductance (uH)

6 T5 24 0

10 T4 34 2.5

12 T4 46 2.5

15 T4 64* 2.5

15 T3 33 5

17 T3 51 5

20 T2 54* 7.5

20 T1 37 10

30 T0 51 16

*Longer whip is preferred, lower SWR.

Here are the pictures: