Because of the very limited space for antenna placement, 20m2 at most, I have experimented a lot with different antennas for transmitting and receiving over the past few years. Most important for me is the performance on the 10 meter to 20 meter band. For those bands, I have now chosen the Cobweb antenna, a multiband round-beam dipole measuring only 2.8 x 2.8m. For 30 to 80 meters I listen with Wellgood loops v4 from M1GEO. But I was looking for another solution for transmitting on those bands and found it in a remarkable vertical, which turned out to do so well from 20 to 10 meters that I wondered if it was equivalent to the Cobweb. Time for an extensive, “semi-scientific” comparison!
The Rybakov vertical for 30 to 80 meters
That particular vertical that also wanted to use 80 meters up to 30 meters for transmitting is a so-called Rybakov 806 vertical. That is a ca. 7.7 m long vertical, fed via a 4:1 unun and tunable with a tuner for optimal SWR. The Rybakov is usable from 80 to 6 meters.
Can the Rybakov do what the Cobweb can?
For the 20 meter to 10 meter band, I experimented a lot with different variants of a round-radius dipole; the Cobwebb, Cobweb and Classic Halo antenna. In the end, the choice fell on the Cobweb. But to my surprise, that Rybakov vertical also performed quite well on those bands. Both with transmitting and receiving and sc hf wrote a blog about that as well. I started asking myself if the Cobweb was superfluous and I could work from 80 to 10 meters with that one Rybakov antenna. It was time for a duel in terms of reception; Cobweb vs Rybakov. Disclaimer: the results are based on the situation at my QTH and may therefore be different elsewhere.
First of all, a disclaimer. The measurement rare not done in a laboratory, but with my own setup, as explained below. I just do not have the possibility to test two antennas in open field with several wavelengths distance. You should therefore consider the measurements and conclusions as a an indication for what it could do for you.
The test setup
First, a few words about the test setup. As a receiver, I use an SDRPlay RSPduo. This is an SDR with two identical tuners, each with their own antenna input. Two identical sound cards (Soundblaster Play3) on one PC handle the audio, with two instances of the latest version of WSJT-X decoding the signals.
I tested SSB, FT8 and WSPR. With WSPR and FT8 providing by far the most objective data and opportunities for analysis.
The Cobweb is a 7 band antenna from Polish builder AWK, suitable for 20 meters to 4 meters. It stands on a mast at 13 meters and is fed by a 1:4 hybrid balun (impedance transformer + common mode choke).
The Rybakov is an aluminum antenna 7.7 m long. It stands on a 9-m mast and is fed through a CG-3000 automatic tuner directly at the feed point, where there is also a common mode choke. A total of six radials over 7.7 m long, 5.5 m long and 2.5 m long are used as counterpoises.
Both antennas are fed with RF287UF 50 Ohm coaxial cable. In doing so, the Cobweb has about 5 meters more coax than the Rybakov. The loss over that length is about 0.15 dB and thus actually negligible for this comparison.
All other antennas were short circuited to ground.
To rule out any possible performance differences between the two receivers, I switched antennas and sound cards daily between the two SDR receivers.
For comparison purposes, I collected data over a period of four months, starting in November and ending in March.
First impressions in SSB
On 20 meters and 17 meters, the Cobweb produced significantly better audible signals in SSB. With the Rybakov vertical, signals were noticeable deeper in the noise. Possibly the result of local QRM. On 15 meters and 12 meters, the two antennas seemed fairly equivalent in SSB, with sometimes the Cobweb and sometimes the Rybakov producing a better signal. But rarely with a big difference.
I did notice that the signal strength on the Cobweb was less susceptive QSB than the Rybakov. Not sure why… Where the signal on the Rybakov sometimes completely faded out, it was still audible with the Cobweb.
Objective measurements with FT8 and WSPR
Since observations in SSB are not objective, I used two other modes: FT8 and WSPR. These modes clearly show the performance of both antennas in signal to noise ratio (SNR). In other words, how strong a signal is received relative to noise.
The possibilities to analyze FT8 reports are limited, but with some creativity I was able to compare figures in terms of the number of “spots” received.
The 20 meter band had by far the most data to collect. In FT8, the Cobweb was able to receive 36% more spots. In WSPR the difference was even greater, there the Rybakov could receive less than 43% of the number of spots of the Cobweb.
On this band, the difference was smaller. In FT8, the Cobweb received 31% more spots than the Rybakov. In WSPR, the difference increased further. In that mode, the Cobweb received over 50% more spots.
Again, the Cobweb wins again with 22% more spots in FT8 and 18% more spots in WSPR. The smaller difference is well explained because on this band the Rybakov behaves like a ½ wave antenna with a relatively favorable radiation pattern.
Here again the Cobweb was better, but received only 15% more spots in FT8 than the Rybakov. In WSPR, the difference was 18% in favor of the Cobweb. The Rybakov behaves like a 5/8 groundplane on this band, with a favorable radiation pattern.
The difference on 10 meters gets even smaller. The Cobweb admittedly wins in FT8 with 13% more spots. In WSPR, the difference is still 10% in favor of the Cobweb. Here the Rybakov behaves like a ¾ vertical, also with a favorable radiation pattern.
I had the expected that due to the height of the 10 meter element in the Cobweb, 13 meter AGL, the Cobweb would easily beat the Rybakov, especially on (multihop) DX signals. The only explanation I have for this is interaction between the 10 meter element and other elements, making the radiation pattern unfavorable. To investigate this, I’m also planning to start an experiment to compare the 7 band Cobweb on 10 meters with a mono band Classic Halo.
Day/night and high flux
The aforementioned differences are averages during daylight and in the dark. What I noticed is that the Cobweb really started to make the big difference after sunset and especially on 20 meters. Where the Cobweb was still receiving multihop DX until late in the evening, the Rybakov was silent. In addition, I also noticed that in weeks of high solar flux, the difference between the Cobweb and the Rybakov was greater.
More backscatter spots
When I started looking at the spots in more detail, I noticed that in general and especially after sunset, the Cobweb receives more DX, but also local spots than the Rybakov. Local as in a radius of about 100-300 km around my QTH. These were often weak signals. My suspicion is that these are backscatter signals. DX and backscatter are mainly signals coming in at low angles. More or less proof that the Cobweb performs better in the low radiation angles.
The Cobweb performs better in terms of reception on all bands. You can hear that even if you listen to signals in SSB and CW with the naked ear, and the spots in FT8 and WSPR confirm that. Leaving the 20 meter band aside, the Rybakov doesn’t do too badly at all. It is a fine, easy and forgivable antenna. If you are not blinded by its performance on 20 meters, you are not making a wrong choice with the Rybakov, especially since it also performs fine on 30 meters and 40 meters as well.
New experimental research 1: transmission performance
Still, as far as I am concerned, the comparison is not yet finished. First, I also want to know how both antennas compare in terms of transmission performance. Because suppose the Rybakov is superior to the Cobweb in terms of transmitting? Testing started in March and April. I expect to publish the results in May or June
New experimental research 2: multiband Cobweb vs monoband Halo
It is a mystery why the Cobweb does not perform superiorly on 10 meters. I suspect the interaction with the other elements is the cause. To do that comparison, I am going to build a monoband classical Halo and compare it with the Cobweb on transmit and receive.
New experimental research 3: height vs performance
I also want to know how a horizontally polarized antenna performs at different altitudes. How does a Cobweb at 13 m height perform vs. an identical Cobweb at 10 m height? Will the Cobweb at 13 m height receive more backscatter than at 10 m height?
To be continued…
Analysis done with www.wspr.rocks en www.wsrp.live online tools.