Radio W4KAZ

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Pollenating the Antennas – 80m Folded Dipole

Spring Fever, Hay Fever, and on the positive side, Antenna Fever.

The antennas are still in somewhat dis-array. But chugging right along in its 9th year of service is the trusty 80m folded dipole. The dipole serving the KazShack is cut according to the cookbook dimensions for folded dipoles outlined in the ARRL Antenna Book. In the ?? edition, the most recent available here, it is the lead off in Chapter 15, Portable Antennas. For those without the Antenna Bible, it was also re-published WITH permission from the ARRL, in the March 2002 edition of the CARC “FEEDLINE” newsletter. See pages 5 and 6.

I didn’t realize it had been in service so long. Given its construction from 300 ohm TV twinlead, I expected the wires to fail. It has required a repair, but that was due to damage from falling limbs. That is a problem with all of the antennas here.

It has occupied two seperate locations. Its first install favored the northwest, and it was really a killer into the upper Midwest, even with just 100w. It currently is oriented favoring a compass heading just to the east of due north. This improved the signals from Europe and the New England states, but is really a compromise.

The wonderful thing about folded dipoles is the hugh SWR bandwidth. The local version favors the CW/80m end, but the 2:1 SWR bandwidth is from about 3580kc to 3800kc. It is still well below 3:1 SWR at the bottom of 80m. It goes above 3:1 at around 3970kc. The internal tuners of both the FT-920 and K2 can bring the match down to 1:1. Sweet.

This antenna is a keeper. The only problem I have with it is that the construction article does not delve deep enough into the theory. I would like to reproduce the antenna using 450 ladder line, but don’t understand the methodology of the matching section well enough to scale the antenna. Hmmm. A conundrum.

One down, one fixed

The 160m-L has been “on the fritz” (i.e., broken) since the ARRL 160m contest. It went crazy when it started raining during the contest. The SWR went high, and it became deaf.

The ground was still covered with snow from our very rare in central NC five inch snowfall. Old XYL tales relate that antennas work best when installed/repaired in inclement weather, so the time seemed right.

It turns out there was a bad connection on one of the jumpers. I took a bit of time to find the problem, and only about 30 seconds to fix. To debug it I plugged the dummy load into the feedline system out at the antenna end. The high SWR into the dummy load told me the problem was in the line and not the antenna. Fixing the line solved the immediate issue.

That whole antenna system needs closer scrutiny. It was intended to be ‘temporary’ when it was hauled up three years back, and the feed line is a hodgepodge of jumpers which were available around the KazShack at the time. If it stays up, the matching network needs to be corrected. At last count, it now has 18 elevated radials. The feed point is about 8 feet off the ground mounted atop the kid’s wooden swing set. The radials are all of different lengths, tied off to trees. The distance of the tree from the antenna center determines the radial length. It seemed to be playing much better with 18 radials than it did in the original four radial configuration.

I suppose I’ll play around in CQ 160 CW, but I expect it will be an all US contest here in the KazShack. 100 watts is effectively QRP when run into a poorly matched inv-L. I’ll find out if I feel like trying to run stations.

The heavy winds from a couple of weeks back knocked loose a rather large branch, which clipped the nested rectangular loop for 15m and 10m. That simplifies the job of fixing it, something that has been much lower on the radio project list.

But without sunspots, I’m still in no rush to get 10m fixed. 15m either, unfortuneatly.

Some Assembly Required

The shack is still in disarray but there is light at the end of the tunnel. The acorns are raining down like hail, so the leaves are not far behind. It is time to wrap up the shack re-assembly before leaf raking(and contesting!) season arrives in earnest. I need to keep the leaves from carpeting the grass, or else the small amount of grass will be croaked by the carpet of leaves.

The long outstanding antenna switch has been finished. I already had most of the parts ready. The remote switch and inside switch box have been assembled for quite a while. I finished the outside enclosure over the summer. During the last couple of weeks I put connectors on the control cables and ran them from the shack to the location chosen for the switch. One of the first really pretty fall days dropped in on Saturday so final assembly of all of the components got DONE! Sweeeet.

That same beautiful Saturday was perfect for performing maintenance on most of the antennas and their support ropes. Time to adjust the 20m and 40m dipoles. They were both too short, and resonant above the SSB segments. Both are now resonant just above the middle of the CW segments on their bands.

The 15m/10m antenna is a nested rectangular loop. It is the same size or length of wire as a normal quad loop, but instead of square, the vertical sides are just a bit less than 2 times the length of the horizontal sides. It is fed at the bottom(a short side, and closer to the ground) and is thus horizontally polarized. My own antenna differs somewhat from this website description and the original idea goes back at least to K6STI. In my own version, I use a length of aluminum channel as the top horizontal. I’m really happy with this antenna. When the bands open, it works well. Its most endearing characteristic is that it is easily rotatable, as it hangs from a single support. It has tag lines, which are used to rotate and secure the direction.

The rectangle also has the benefits of any quad loop while showing a nice 50 ohm match. Sweet. I had been feeding each loop from a single feedline. Since I’m planning ahead for the use of the band pass filters, it was time to put each on a separate line. Since the antenna needed to be dropped anyway, I also put up a set of fresh ropes.

Each loop required re-pruning after they were separated. Both required about four inches of wire added to bring their resonances down to the CW band segments. They also now show more a broad coverage at less than 2:1 SWR. Using a single feed line is a bit of a trade-off. You trade a bit of bandwidth for the convenience of a single line.

Since they are now using separate feedlines, I could have gone to normal equilateral quad loops, and maybe used a balun or quarter wave matching section. It would have raised the average height above ground. This was a bad idea for my situation given their location, because the increased length of the horizontals would make them more of a snaggle problem with adjacent trees.

I’ve also got the K2 160m module installed, so I’m looking forward to that. Band Pass Filter Fever is on hold until I can scrounge up enough parts.

In a very unusual turn of events, Murphy was off bothering someone else Saturday. 🙂

Rohn Tower Trades Hands

Picked this up off theTower Talkreflector. Rohn Tower has been sold by Radian to a company in Peoria called O’Brien Steel. Radian has themselves been recently acquired by Montreal based Prestige Telecom.

So ownership of Rohn returns to Peoria. I hope that is good news for the employees as well as their niche market customers – hams.

Field Day Antennas 2008

This year will probably be a Vee beam for 40m, fed with ladder line and a tuner. Interesting factoid: Vee beams that are odd multiples of 1/4 wave seem to have impedances similar to loops, and can be fed with the standard quarter wave matching section, although that will limit their utility to a single band.

80m will probably be a dipole, 20m is uncertain. The Vee may be pressed into 20m service, and will be used if there are decent 15m or 10m openings.

Throw down a comment if you have a favorite.

Amended 6/4/08: Ok, maybe a loop on 20m. Probably diamond shaped and fed at the bottom.

Hexbeam Broadbanded

I was really hopped up when I ran across the web site of G3TXQ. I was particularly hopped up by his documentation on his hexbeam experimentation, particularly the modified geometry broadband version.

I played around with my own experimental hexbeam for 17m/15m/10m, back in 2000-2001, right after I had re-licensed. There’s no magic to the antenna, but it does give one the performance of a 2 element yagi in a much smaller physical space. The hex design is right at the top of my “really interesting stuff” list. Its one of the things I corresponded with W4RNL over. ‘RNL was not a great fan of the design, but agreed that it did have its place. I’m not sure of the reason for my own illogical interest – just one of those things.

But back to G3TXQ. He has taken the step of modifying the geometry of the reflector. He indicates that this change will improve the f/b ratio and also gives a broad match that is closer to 50 ohms. The classic Hexbeam design has an impedence in the 20 to 25 ohm range. The G3TXQ version has a flat SWR across the 20m through 15m bands, and most of 10m. Way Cool. Even better, the modeling shows the best SWR and best f/b ratio seem to happen at approximately the same frequency.

I’m thinking about a 15m monoband version now. I guess its also time to go back and surf the hexbeam yahoo group again.

More N1LN Antennas Flying

N1LN is still chipping away at flying all of his antennas. Two more went up on Saturday for the 20 meter stack. This has been a fun project to watch go together, as I’ve not been involved with anything of this magnitude before. It is probably not so much fun for Bruce yet-he and Laurie are doing 99 percent of the work themselves. He still has a great deal of work outstanding, but it is really coming together for them very well, and they have planned for it for a long time.

I you want to see the photos, continue to the full page.

Continue reading More N1LN Antennas Flying

New Ears – Addendum

Wow, the relocated, refitted, and repaired 40m and 80m antennas really sound a lot better than before.

The 80m folded dipole is also now almost perfect from an SWR standpoint. It is under 2:1 from 3550kc thru 3850kc, and the rig’s autotuner can easily match it across the rest of the band. The low SWR sweet spot is about 150kc wide, and centered on 3700kc. This antenna was constructed per cookbook dimensions from the ARRL Antenna book’s chapter on ‘portable’ antennas.

I’ve also worked out a switching arrangement for 20m and 160m, which both require an antenna tuner. 20m is a ladder line fed dipole, and the 160m antenna is a 160 foot long inverted-L with about 70 feet vertical. I have the inv-L routed through my jumbo homebrew tuner to an MN-2000, and the 20m direct to the MN-2000 through a balun.

I expect to add a 20m vertical soon, for a just-in-case backup/alternative. I also expect to find a way to incorporate a homebrew antenna switch to reduce the number of feed lines snaking through the yard into the basement.

But Biological Antenna Supports are still holding on to their leaves, so I have many hours of maintenance ahead.

New Ears

I had a few minutes to spent playing around with the new antennas, just listening around. The guys calling CQ for the Pennsylvania QP were booming in here on 40m during the afternoon. The 40m dipole really seems to be hearing well.

On 80m, the new antenna was showing a very high noise level across the band, and it sounded similar to the AGC pumping from a strong nearby station.

“Hmmmmm…” thinks w4kaz, eyes vacant and scratching head idly.

Turns out my 80m antenna is now flat broadside to the WPTF 50KW antennas that are about one mile to my NNE. I have an 80m bandpass filter I built a couple of years back. Plugging that into the line cleared up the WPTF problem, dropping the broadcast QRM from S7 to S0. Its still there, but its very low level now. Not many signals on in the afternoon, so I need to carve out some time to spend in the shack playing around some.

Lesson: 50 KW at less than one mile distance is undesirable. (Yeah Homer, I know….doe-OH!)

Biological Antenna Support Structures

I spent much of the day tugging on lines and shooting new lines up into the trees. We had a really blustery day back in the late spring that knocked a lot of the weaker branches out of the trees. As luck would have it, one of those smacked the 80 meter folded dipole, snapping the light cord I was using to hold it up.

That was a bit of a problem. The line I had placed for 80 meters was probably 80 feet up, although the antenna was only up at about 50 feet. With the leaves already back on the trees, I couldn’t shoot a line back to the same spot, which would have been my first reaction to the problem. Instead, it forced me to think a little harder. (Queue soundtrack of rusty hinges squealing….) How best to make lemonade from the lemons?

Continue reading Biological Antenna Support Structures