04-22-2012, 11:56 PM
Congratulations, Lars; It looks good and glad you snagged some nice DX so soon (and at a low height).
We are now underway in our move and will be flying out of Hawaii day after tomorrow. Expect to arrive in W5 after a week or so of driving from the West Coast with stops along the way.
Am using wi-fi for the first time today and it's great; I'll be able to check out this and other threads in the forums periodically, too.
Aloha from Hilo, Hawaii (only 25 miles down the hill from the old QTH but we're all packed and travelling now!)
04-25-2012, 05:51 PM
Tnx Jeff. Good to see that you found another field where wireless makes sense ;-)
Good & safe traveling!
I have got a question for you or anyone else:
I built my Cobwebb using 2 meters long fiberglass tubes. The means I have a little bit of length left when the 20m dipole is added. I was considering using this bit to add a 6th wire for an electrically shortened 40m dipole.
Although I have never designed my own electrically shortened dipole I think I should be able to manage with the information available on the net.
What I do not know and maybe someone else does:
1. can you fold an electrically shortened dipole AND assume that it behaves (resonance-wise, not performance-wise) like a folded full size dipole?
2. can you add an electrically shortened dipole for 40m to the existing antenna and expect the antenna to behave as it does today on the higher bands?
I can of course experiment with it by the time I get my Cobwebb installed but it would be nice to know of any theory or practice that could help me decide how to design this or to better not do it in the first place.
73, Lars / PH0NO
04-25-2012, 07:24 PM
By "folded" I assume you mean "bent" rather than a 2-wire folded dipole.
If so, one problem I see is that the feedpoint impedance of the loaded (shortened) dipole will be even lower than the 12 Ohms of the "full-size" bent dipoles. Your SWR will likely be high on that band, and there may be dissipation issues for the 1:4 balun. Finally, depending on how you intend to load it to resonance, you could find it presents a low impedance on one of the other bands and effects the SWR there.
There are only so many quarts you can squeeze into a pint pot
04-25-2012, 09:51 PM
Thanks for you quick reply Steve.
You are right. I mean "bent" - as in Cobwebb-style, not the folded dipole you discuss on your website.
I understand the point you make about the low impedance it might present on other bands. That will be a challenge. Even more so as I don't have the tools to measure what is going on.
The point about the shortened dipole itself having a lower impedance is something I cannot fully grasp and this irritates me enough to read up on radiation impedance. [time lapse] I am beginning to see the light..
While reading up on this subject I also found remarks about shortened dipoles being more susceptible to coupling issues with nearby / parallel conductors. That might also become an issue here.
Knowing all of this I will stick to my sloping end fed wire for 40 & 80m until I find another design that is more effective but still low profile.
Thanks a lot for sharing your knowledge.
73, Lars / PH0NO
06-17-2012, 04:27 PM
Lars; I wonder if it would be feasible to try a full-size 40M cobweb? Monoband, of course---probably would be just under 17' square so probably not out of the question with the right lengths of fiberglass.
I would be tempted to try one myself except for my inclination to operate the higher bands 99% of the time.
I'm now setting up my ham shack at my new QTH here in New Mexico and have started reworking my cobweb. The balun got a good soaking in the rain before we left Hawaii so I am cleaning all the rust and corrosion and remounting the connection points (ground bus bars) deeper so the cover will seal effectively this time around.
I've also decided to use non-insulated wire for the elements as the UV is so strong here with our continually sunny skies that I know it would be a short time before the PVC insulation would be lost---so am going to try bare stranded copper (18awg) instead.
Making slow progress with so many other things to do after making such a big move but will post my results here once my "new and improved" cobweb is up in the air sometime in the near future. It will be great to get back on the air after a few months otherwise.
06-17-2012, 08:41 PM
Mine has been sitting on top of this RV on for a continuous period of 8 months now while the RV was parked. I took it down the other day so I could drive the RV IMAG0161.jpg
I would like to share some of my observations.
The folding tripod on the roof has 3 aluminum tube sections that fit into one another which supports the cobwebb on the RV roof.
I am using dacron rope as guy lines. They are tied from the antenna hub to the 4 corners of the RV on the roof rack / railing.
Firstly, this antennas has been though high winds, a few hail storms, and has even iced up from freezing rain.
I have a rig in the RV and this antenna has performed flawlessly. I have made many DX contacts using the antenna installed about 25 feet off the ground as shown in the photo mounted above the RV roof. Some of the more notable contacts off the top of my head were made with stations located in Hawaii, Alaska, Europe, South America, Africa, Australia and Japan.
The VSWR has never changed since installed and it provides good bandwidth. When I took it down I inspected it closely. The UV resistant wire ties have not yet become brittle. The crappie poles are now showing UV oxidization on the top of them. They have the appearance they are "sun scorched" but the sides and bottom of the poles are still nice and shiny. The poles can still collapse just fine and no water was found trapped inside them.
The feed point box I am using was taken apart and inspected. There doesn't appear to be any signs of corrosion or signs of trapped moisture occurring inside the feed point box. All the internal copper wiring still appears bright and shiny. I did however break the 10m element when taking it down. The wire "tore" away from the feed point box due to tension I created on the connection during disassembly.
Not a real issue since I can reconnect the wire but I feel it was due to the fact I am using relatively thin antenna rotor wire for the elements. I used this type of wire for two reasons, first because of the high UV resistance outdoor rotor wire offers and secondly because the wire is lightweight. . In other words, the wire doesn't have a high mechanical strength and can break when pulled hard due to tension. The connection however did hold up well when the rotor wire was covered in ice. The rig I am using is 200 watts and the TV antenna rotor cable wire seems to handle that power level on any band without any issues.
The hub assembly is working out quite well. There are no signs of corrosion or cracks in the welds. When the poles were removed from the hub, there was no trapped water inside.
I have the entire antenna inside a storage bag now intended for portable operation. It all came down and was disassemble and stored inside its storage bag in about 15 minutes. However I am concerned the elements need special consideration when storing them. I was thinking I need to prevent them from tangling with one another. I might devise a setup which the individual elements can be removed and stored seperately inside film cans or something. The Dacon holding the element ends together are holding quite nicely.
Now I am wondering if the antenna will maintain it's same VSWR and bandwidth when it goes from it's storage bag and back up on the pole. I am feeling it might be finacky in that department when used as a portable antenna arrangement, but for permanent installation the Cobwebb has been a stellar performer and I am very pleased with my results. It certainly has performed much better than a dipole and even verticals in my A/B testing. It is a very quiet antenna and it sure does bust the pileups very nicely. Most people could not believe I was setup as a portable "mobile" station. I was more often than not around 20+ over S-9 to a whole lot of people using this antenna. They were often blown away when I told most people how I was just using a mobile setup. While I realise this is not a directional antenna, it felt and operated a whole lot like a Hexbeam to me in terms of instant, on the fly bandswitching experience without the need to use an antenna tuner, overall signal reports, low noise characteristics compared to other antennas and general performance.
Well that's all I have on the Cobwebb for now.
73 de Charles - KC8VWM
North American QRP CW Club #3159, SKCC# 5752
06-18-2012, 12:37 AM
You've built it upside down. The junction box is supposed to hang BELOW the arm. That way when water gets into the box it doesn't sit in the choc connector strip and turn it into a pile of rust.
Originally Posted by MI6KAK
06-18-2012, 05:18 AM
True, he has built it "upside down" but the reason so much water got in mine was due to a bus bar being mounted too high and preventing a proper seal in the first place. I don't think mounting it hanging as I did or upside down facing up really made the least bit of difference---as long as the balun box is properly sealed in the first place the installation should be fine.
Ironically, where I am about to use it is so dry (6% humidity vs. 100%) that it won't matter much 95% of the time. It will matter come monsoon season and some of the time in winter when it snows here so I will endeavor to make it as waterproof as if I was still living in Hawaii...
Nice to read about your reassessment of your cobweb installation and am glad to see that you're as enthusiastic as ever about your results, Charles.
Although we found a place that is essentially antenna friendly it's still in the middle of town with lots of overhanging tree branches and I'm going to be somewhat limited in what I can put up the air here at home. That means I will put my 6M Yagi up along with my cobweb for 10 thru 20M. Probably will add a 2M Yagi and maybe a 6M loop eventually to round things out for what I do with weak signal work but that should be the extent of my new antenna farm---maybe not as extensive as my previous QTH but I'm not complaining one bit.
I plan to take some more pix once the antennas are up and will post a full report thereafter. I believe that I will be as enamored with the cobweb as much as Charles is once it is finally and properly tuned (without a balun box full of rain water!).
73 to all, Jeff now in DM65
06-18-2012, 04:47 PM
Originally Posted by NH7RO
Nice to see you are almost back in business.
I still haven't put up the Cobwebb as I haven't been able to design a feasible low profile antenna mast yet. As we are also considering putting up solar panels (that would be in the way of any mast construction I was thinking of vice versa) I am not sure it will ever get a fixed place at my house.
On the other hand, I have now constructed a very solid mast for portable operation - one that doesn't need guying and should go up to 30ft (still need to add one segment for that height). I have done this mainly with my new 10m yagi in mind but I will definitely use it for the Cobwebb as well. Would be nice to see if it gets me across to NM (and how it compares to my usual /P antenna: a vertical end fed wire).
Will keep an eye on this thread to see when you are up and running again.
73, Lars / PH0NO
06-18-2012, 06:03 PM