Tuning the Cobwebb antenna with a different approach

The Cobwebb is an antenna that has gained in popularity in recent years. The horizontally polarized omnidirectional antenna is suitable for the 10m, 12m, 15m, 17m and 20m band. All elements are fed with a single feed line. At only half wave length above the ground, it will outperform any vertical antenna. Measuring only 2.8 by 2.8m, the Cobwebb is the solution for the DX’er with limited space. But the Cobwebb design has some disadvantages: it’s lack of bandwidth and tuning it to a reasonable SWR.

The three variants of the Cobwebb antenna

The Cobwebb antenna comes in three variants:

No.1: The G3TPW from Steve Webb is the original design of the Cobwebb. The elements are folded dipoles with open ends, usually made from figure 8 speaker wire. Exactly in the middle of the folded dipoles, the two parts of the dipole are interconnected. This shorting point creates a kind of T-match to bring the impedance to 50 ohms.

A Cobwebb original design antenna. Notice the the short circuited points about halfway each element. Image from G4FZQ.

No.2: The G3TXQ variant from Steve Hunt (sk), also called Cobweb (with one letter ‘b’), uses open dipoles, single wires. The impedance is brought to 50 ohm by means of a 1:4 balun. It is the most commonly built or purchased Cobweb.

The Cobweb used single wire open dipoles. The feedpoint box holds a 1:4 balun and common mode choke. Image from G3TXQ.

No.3: I call the third variant the Haloweb. It uses closed-ended folded dipoles, also from figure 8 speaker wire. This is the same principle as the classic Halo antenna, known from VHF. The impedance of this variant is 50 ohms and therefore does not need matching. Thanks to rhe closed-ended folded dipoles, the Haloweb offers about 15% more bandwith over the G3TXQ Cobweb.

The Haloweb uses folded dipoles with closed ends for increased bandwith.

Haloweb, the best of both worlds

I built all three variants. They are almost identical in construction. The G3TPW is the most challenging in terms of tuning. That is because in addition to tuning by means of the lengths also the shorting points have to be moved. The G3TXQ is a lot easier to tune, but in terms of bandwidth the G3TPW offers about 15% more bandwith. The variant with the best of both worlds is the Haloweb. This Cobwebb variant combines the bandwidth of the G3TPW, with the ease of tuning of the G3TXQ. In the end I stuck with the concept of the Haloweb.

Choose between Phone and CW section of the band

The bandwidth of the Cobweb (with the one letter ‘b’) is limited. At 20m and 15m it is about half the band and at 10m no more than 500kHz. When tuning to 20m, for example, you have to choose between the “Phone” and “CW” section of the band.

A test of your patience

There is another ‘challenging property’ to all three variants. That is the 50 ohm adjustment of all elements when using the 5-band version. This is truly a test of your patience. With an MFJ-259B analyzer in hand I managed to get all bands resonant, but in no way did I get the SWR to an acceptable level < 2:1. I only got the element for the 20m with all variants with ease close to 1:1. All other elements were all above 2.5:1.

My challenge with multiband antennas

Now I have always had a at my location. For example, I was never able to get my Hustler 5BTV trap vertical to an acceptable SWR on 15m band. I also never got a homemade WARC vertical for 12m and 17m on both bands, at the same time, on a decent SWR. It will be my location. Possibly a piece of metal in my home is the cause.

The best way to adjust impedance

Since I got all the elements resonant, I thought it was a shame to not use those bands on which the SWR was bad. The solution is to tune each element separately for those bands, for example via the built-in tuner in your transceiver. But that results in a lot of loss in your coax cable. The best way to match impedance is to do it directly at the feedpoint of the antenna, not in your shack.

CG-3000 automatic antenna tuner

Now I have worked with a 13m long vertical antenna for a long time. I used to tuned it with a CG-3000 automatic antenna tuner directly at the feedpoint the vertical. With the dismantling of this antenna, to make room for my Haloweb, I mounted the CG-3000 a floor lower with a piece of wire as antenna, but that was not very successful. Lots of QRM and poor radiation. Guess what? That CG-3000 remote tuner could well be used with the Haloweb.

The CG-3000 autotuner tunes form 160m to 10m handling 200 watts

The tuner in the construction of the Haloweb

With some pieces of aluminum from my scrap metal collection, I made some brackets and mounted the tuner where normally the feedpoint would be mounted. The tunew weighs about 2kg. So to save some weight, I replaced the folded dipoles with open dipoles, initiating the Cobweb variant. The advantage extra bandwidth with folded dipoles is namely largely canceled out by the tuner, so I presumed open dipoles would do fine. When determining the wire lengths, I took the length of the internal wiring of the tuner into account.

The CG-3000 remote tune at feedpoint of the Cobweb antenna.

ZL with 25W FT8 on 20m band via long path

As I expected, I could now tune the antenna to any frequency on 10m, 12m, 15m 17m and 20m. After about four weeks of testing with WSPR and FT8, I could not notice any significant difference in performance compared to the Haloweb. For example, ZL with 25 watts FT8 on 20m band via the long path, went quite easy.

ZL via the long path end of April 2021 using 25 watts in FT8.

Surprised me in a positive way

I found the combination of Cobweb antenna with autotuner, a successful setup to solve my impedance matching issue. It also allows me to work each earlier mentioned band entirely. I only need to retune when it’s close to band edges. In terms of performance this setup surprised me in a positive way.

Easily tune it on all other HF bands

Although the Cobweb antenna is not built for other bands, the CG-3000 can also easily tune it on all other HF bands. The Cobweb did surprisingly well at 30m. Even on 160m band with 25 watts I was received in more places in FT8 than ever in the past with the 13m long vertical with the same tuner.

The Cobweb tuned to 160m band outperforming the 13m vertical

Test with folded dipoles for less noise?

Nevertheless, I am also going to test with the folded dipoles. These would be less sensitive to manmade QRM than the open dipoles. I have plenty of manmade noise to test it in practice.