A different approach to tuning the Cobwebb antenna

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 through one 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 narrow bandwidth and tuning it to a acceptable 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 4:1 balun. It is the most commonly built or purchased Cobweb and commercially available variant (like MFJ, EAntenna).

The Cobweb uses single wire open dipoles. The feedpoint box holds a transformation balun combined with a a common mode choke. Image from G3TXQ.

No.3: I call this third variant the Multiband Square Halo, based on the classic Halo design. 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 the closed-ended folded dipoles, the Square Halo offers about 15% more bandwith over the G3TXQ Cobweb.

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

Square Halo, 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 element 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 probably the best of both worlds is the Square Halo. This Cobwebb variant combines the bandwidth of the G3TPW, with the ease of tuning like the G3TXQ.

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. You would need a tuner to increase bandwith with an acceptable SWR.

A test of your patience

There is another ‘challenging property’ to all three variants. That is the 50 ohm adjustment of all elements to resonance and SWR 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. In all variants I only got the 20m element resonant with a good SWR. All other elements were all above 2.5:1 SWR.

My challenge with multiband antennas

Now I have always had a ‘tuning challenge’ 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 adjusted to a decent SWR. It must be the location. I suspect that some metalwork in the roof of my house 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 because 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, especially on long runs. The best way to match impedance is to do as close as possible to the feedpoint of the antenna.

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. This antenna was quite a good performer in transmitting from 60m band up to 17m band. But the local noise was always awful. As a receiving antenna it performed rather poor.

With the dismantling of this antenna, to make room for my Square Halo, I had the CG-3000 remote tuner laying in the shack, doing nothing. So I decided to put it up in the Halo and experiment with tuning that antenna.

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

The tuner at the feedpoint of the Square Halo

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 tuner itself weighs about 2kg. Not too heavy to put it on the boom. The extra bandwidth with folded dipoles is canceled out now by the tuner, so I presumed open dipoles would do fine and would reduce total weight. So I stripped the folded dipoles to open dipoles. When determining the wire lengths, I took the length of the internal wiring of the tuner (approx. 20cm) 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 Square Halo for 20m, 17m and 15m. For example, ZL with 25 watts FT8 on 20m band via the long path, went quite easy. But it seems that 12m and 10m are performing less good in RX, however it’s a gut feeling still. Is suspect that radiation pattern is disturbed on these bands. PA0JBB Hans mentioned in a post that the power supply cable of the tuner could be participating as active part of the antenna. Something I need to research.

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 tune it on all other HF bands as well. The tuned Cobweb did surprisingly good 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 auto 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, some say… I have plenty of manmade noise to test it in practice.