VHF/UHF X-Beam Construction Notes (and HF see update below) -- k0emt
I sat down and drew out what I did and thought I'd pass on what I did/found out. The design is from The 1993 ARRL handbook that was given to me by wd0hbx. The x-beam was originally intended as an HF mono-band beam. I used the formulas to build it for 6m. It also happens to work out really well for 2m & 70cm use as well.
The text says that this antenna design will typically have a 6-8dB gain and a forward to back ratio of 20dB. The handbook also states that this design should perform as well as or better than a similar yagi.
(That will make more sense when you look at a picture.)
There are tails on all four arms.
I didn't have my arms exactly perpendicular.
I had the inside angle of the directed element < 90 degrees.
I used square tubing on the back and round on the front.
Tie the tails to the nylon cord that is strung around the beam for support.
f swr (48.4 - 51.35) <= 2.2 50.110 2.2 420.000 1.3 50.125 2.2 426.000 1.6 52.525 2.5 430.750 1.6 435.000 1.1 144.000 1.1 436.300 1.6 145.050 1.2 442.000 2.0 146.000 1.3 444.000 1.6 146.400 1.4 445.000 1.1 146.520 1.4 446.000 1.5 148.000 1.2
On air testing has shown that it is indeed directional. Had S-8 noise on discone, switched to beam, now S-3. Conversely, when testing on 6M with a horizontally polarized "local" station kb0qhz, noted an improvement of 5 S-units when using beam. (cross polarization could come in to play though -- the discone is vertical.)
When testing with kb0hnr mobile in Jefferson City, MO I was able to copy him at times S5-9 when the discone couldn't even hear him. A similar gain was noted when hearing DX when cross-polarization shouldn't be an issue.
Noted, that when I changed the direction it is pointed wasn't able to bring up certain local 2M FM repeaters.
Hooked up an ICOM 706mkII, using 20w on 2M SSB made a contact with K9IMX (Lake of the Ozarks), he reported me 57. He regulary works n9wyx, another "serious" 2M ssb station within ~1/2 mile of my QTH and he said he was impressed with my signal.
As a side effect, it seems like the discone is peforming better now than before. The discone is mounted off the top of a 20' mast. The beam is approximately one foot under it. Maybe, it's benefitting from some sort of ground plane effect?
Overall, I'm fairly happy with this beam. I have the feeling I will be building another one this year. I'll use better materials and be more exacting on the next go around.
If anyone decides to build one, please let me know. I'd be glad to help any way
that I can and to stick it on the analyzer when you've finished construction.
2001 Jan 23 *******
kb0hnr was over recently. We had the antenna hooked up to my Kenwood TS-570S. The 570's tuner matched the rig to the antenna on 20 and 40 meters. We had a 59 contact into MA on 20m and were able to check into the 3905 net on 40 meters.
Not bad for a VHF/UHF antenna!
2001 May 31 ******
I now think that the reason the antenna was working on the HF bands was because it was so close to ground that there were some very strange interactions going on. The antenna was on a ground mounted 5ft Rohn tripod, with a 5ft Radio Shack Mast, Rotor & 4ft PVC mast attaching to the antenna. 2M and 440 were exhibiting good SWR but I was having a heck of a time with 6M. 6M would however tune.
Presently, I have moved the antenna a top a ~25ft Rohn 25G tower. As currently configured I have a 1.1swr for 144.2 and a 1.8swr for 432.1. 50.125 is currently showing a swr of 8.3, Rs=8 and Xs=17. Just this last weekend I added a 10' section to the tower increasing it to 25ft. I believe before I had done this that my swr was < 2.0 on 6M and on 2M,440 it was >1.5 but <3.0. I am coming to the conclusion that this antenna is very sensitive to it's height above ground.
For now, I plan to leave it as is, (I don't climb), and use the internal tuner of the TS-570S to make up the difference. Which it does just fine. By summers end I hope to add another two sections to the tower and a hazer. At that point in time I will try to adjust the antenna or make a matching circuit.
As an aside, the 570 will now tune the beam for 10M.
2001 October 09 ******
Beam is now at ~50ft. It is performing well.
Frequency and corresponding SWR as measured with MFJ antenna analyzer.
I could not get my Kenwood TS-570SG to force feed the X-Beam on 28.337.
It did match it on 29.6. The rig will also not make the match on 40 Meters.
Obviously it won't make the match on 80 or 160 either.
2003 April 03 ******
It is VERY important that the arms be of a significantly larger diameter than the tails. More info on specifics can be found at LB's (W4RNL) page below.Here are some additional web resources for your perusal.
Bryan - k0emt