Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC8VWM, Nov 14, 2011.
For such a small foot print, it sure is big on performance. ...What a fun little antenna.
Original design plan specifications vs. measured resonant frequency vs. observed VSWR readings.
Formula developed for antenna rotor wire in cobweb antenna configuration
VSWR = 1.1:1 @ 22.100 therefore x = antenna formula
x = 22 feet X 22.100 Mhz
band..design length.....frequency..actual observed VSWR
10m = 34' 486.2/freq = 28.600 1.3:1
12m = 19' 486.2/freq = 25.590 1.2:1
15m = 22' 486.2/freq = 22.100 1.1:1
17m = 26' 486.2/freq = 18.700 1.2:1
20m = 34' 486.2/freq = 14.300 1.1:1
New antenna design parameters. Based on formula.
band.. design frequency...rotor wire length
10m = 28.450 205.1" or 17.09'
12m = 24.960 233.7" or 19.47'
15m = 21.362 273.1" or 22.75'
17m = 18.140 321.6" or 26.8'
20m = 14.285 408.4" or 34.03'
Changes to original plans design.
10m = add 1.1" (optional)
12m = add 5.7"
15m = add 9.1"
17m = add 9.6"
20m = add 0.4" (optional)
In many ways these antennas have many advantages over a dipole such as...
The`antenna pattern speaks for itself. No more nulls and actual gain is achieved over a dipole in certain directions. Maximizes radiation pattern characteristics. Gain over a dipole in many certain directions is achieved without requiring a directional antenna or a rotor.
They take up far less space than a conventional dipole, but yet remain incredibly efficient. In fact, it's about the same width as the vehicle I have it mounted on, yet the elements are full sized half wave dipoles.
It outperforms a resonant mono band dipole in a majority of the situations when directly compared to the dipole mounted side by side at the same height. The dipole was noisier and the cobweb antenna could hear signals down in the mud the dipole simply could not pick up. Also, other times stations which barely existed on the dipole due to nulls in the dipole pattern were very strong on the cobweb. It was not by a slight margin over the dipole either. It was a complete night and day difference like turning a light switch on and off. This is probably due to the omni directional characteristics of the cobweb when compared to the dipole installed at the same height.
Cobweb compared to a ground mounted mono band vertical? .. Some people swear by verticals, but I am never going back to a ground mounted vertical again, unless I need to use one as a dummy load.... Not even worthy of comparison. Enough said.
Instant on the fly band switching and no antenna tuner required. I will post actual readings later but after spending a little bit of time trimming a pruning, these antennas provide an excellent VSWR and great bandwidth. With the exception of 10m, most band edges are in the range of 1.5:1 or less. Quite usable even at higher power levels.
Folded dipole DC grounded design exhibits natural static buildup discharge capability and low receiver noise by design. The signals sounded much "cleaner" on the cobweb compared to a dipole. It's kind of like there was an FM quieting type of effect when switching between the dipole and the cobweb.
Narrow banded high Q receiving characteristics eliminates wide band junk from other signal sources on other bands from getting into the receiver. For example, image junk from nearby AM broadcast stations are a non issue.
Perfect field day, DX'pedition or portable QRP antenna. Doesn't even require trees for support. Lightweight to carry and quickly breaks down into small package size for storage and travel. Sets up quickly. From the time I take it all out of the storage bag, set it up the tripod, add mast poles, secure the Dacron guy lines etc. to the point of plugging the feedline into the back of the rig takes less than 15 minutes.
I found they are in many ways easier to setup than an inverted V dipole because you don't need much space.
Considering the addition of a 40m element. This would require me to reinstall some of the fiberglass pole sections back into the existing fishing rod poles. So the overall foot print would be larger though. Probably make it into a 12' square instead of 6' square sort of thing. I don't need to modify the feed point box because I could combine the 40m feed with the 20m element holes and attach it to the same point on the internal electrical block.
Could also add a 6m or 2m stiff aluminum wire halo antenna to the screws located on the bottom of the feed point.
Other observations about the cobweb design:
I found it doesn't matter where the wires are actually located on the spreaders that much. I found it's best to simply locate the wires as far away as physically possible from one another. This would be similar in context to ensuring there is plenty of separation between the wires when constructing a fan dipole. Interaction seems to occur when the antenna element wires are located too close to one another. One solution is to extend the rope at the end gaps which will make this possible.
Tuning the antenna:
Speaking of end gaps, I didn't notice much of an effect spacing them 3" apart or 7" apart. I think the main objective is to ensure the ends are spaced no less than 3" apart. Also if the ends are too close together, I am suspecting corona energy might occur across the empty space when using high power. I have spacings ranging from 3" to several inches. Again, the objective is placing the wires on the poles with maximum physical separation from one another.
The antenna elements don't have to be installed as tight as guitar strings either. In fact, little difference in VSWR occurred even if the antenna wire elements were heavily sagging or out of proportion with one another. However, if the wires are too tight, this may bend or flex the spreader poles out of their intended positions.
Some articles suggest tuning the elements by folding the ends of the elements over itself. I didn't notice this really had much effect. A slight shift in resonate frequency perhaps occurred but tuning the elements is best achieved by simply cutting and pruning the ends the old fashioned way as usual. I left about 1" folded over itself on each element at the ends mostly for the purpose of providing and maintaining mechanical strength.
Making the antenna element ends:
Simply fold over 1" of wire making a loop on both element ends and secure it with a zip tie.
Tie a piece of Dacron rope through the antenna wire loop.
Install the rope by pulling the Dacron rope to achieve the desired distance of the element ends from one another and tie a knot at wire loop.
Good job on the thread Charles! Should help a lot of people attempting these antennas. I found when using PSK, the live PSK reporter site shows how omminidirectional the Cobweb is, and how well it works.
I have often heard it expressed that if you wish to achieve performance, "It's hard to beat a dipole antenna."
Hmmm... Maybe this design somehow demonsrates we can?
This was my description earlier
I have created a few video demonstrations of the Cobweb vs the Wire Dipole installed on the same mast just below the Cobweb antenna.
The rig used for this test is a Kenwood TS480HX.
ANT 1 as shown on the rig display is the Cobwebb Antenna.
ANT 2 is the Dipole Antenna.
What more can you say!
Yup, I hope so. The detailed construction photos should help...
Actually I am surprised with all the restricted space limitations people reportedly encounter these days here in the USA, that this antenna design isn't given more serious consideration. Heck it's so small a person living in an HOA can install this "full sized" performing antenna inside their attic.
I never heard of this antenna until recently, so I thought I would give it a go myself and I am actually quite pleasantly surprised with the actual results.
I am left asking myself why this particular antenna design is not more widely used when it clearly performs exceptionally well for it's small physical construction and size.
I admit, I have strung up many wire antennas in the past of much longer and larger size, but clearly this one in particular is really quite amazing. Heck I was even hearing stations in Japan on 10m with it the other day.
No, I am not into magical antennas or anything like that and I am always skeptical of antennas that claim a small size but yet exceptional performance but sheesh, I can't seem to believe it myself how well this antenna design actually works. It's simply a full sized dipole design but with an interesting twist. I am just extremely surprised it's not way more popular than the G5RV's and the Carolina Windoms out there. In my own experience, it certainly performs much, much better then any of those designs. For starters, it's certainly more quieter on receive as my videos clearly demonstrate.
I would like to take this opportunity to express my many sincere thanks to Steve, G3TXQ for his expert technical assistance during the constuction phase of this antenna project during the course of this QRZ.com forum thread and also Stephen R Webb, G3TPW who I consider as the originator of the cobwebb antenna design.
To learn more about the original G3TPW cobwebb antenna design, I encourage you to visit his website: http://www.g3tpw.co.uk/
Thanks for the kind comment!
During the week I tried the idea of adding a second "stagger-tuned" 10m element to my single-wire design to extend the SWR bandwidth. It worked well, but tuning was tricky with considerable interaction between the two elements. I didn't have the patience to arrive at an optimum solution, but the "double-dip" SWR response was very evident.
Glad you're enjoying the antenna. I think one of its major attributes is that it's so small and light that you can get it up to a reasonable height on a single inexpensive support.