Note:- if you are interested in building an antenna similar to this design, please see my other page which details an improved version G8JNJ - Broadband HF Vertical Antenna
Also note that this antenna is NOT the same as the Diamond BB7V which uses a different matching principle.
Some time ago I started investigating broadband antennas that could be used without a tuner. My initial experiments were based on the principle of using an 8m vertical, with a 4:1 impedance matching transformer at the base. This presents a reasonable match on most HF bands, but it still requires a tuner to bring it within acceptable operating limits for modern HF transceivers. This problem can be overcome by adding a 3dB high power attenuator on the input of the matching transformer. However the resulting losses due to the resistive attenuator, and the impedance mismatch it is masking, results in very poor performance on several bands. Typically 6-12dB worse than when being fed with an Auto-Tuner.
These tests made me wonder if any of the commercial broadband antennas would work any better ?
Recently Iain, VK5ZDB, posted some interesting information regarding the internal construction of the Comet CHA-250B broadband vertical antenna. The notes can be found here.
I decided to build a copy of the balun in order to better understand how the antenna operated. Details can be found here.
Lots of correspondence was exchanged with Iain, most of which he has documented here.
Photo of original Comet Balun My copy
My conclusions at this early stage were that the commercial model used a lossy 6:1 impedance transformer, which was unlikely to produce much better results than those obtained with my 4:1 balun and attenuator. However as can be seen from the chart such designs can work reasonably well on some frequencies, and would permit contacts to be made, but at a level of approximately 1-2 S points (6 to 12dB) below that which could be achieved with a similar sized resonant vertical. Optimisation of the antenna and balun may produce slightly improved performance, but it is unlikely to be noticed on the air.
In order to test my version of the 6:1 transformer on-air I used a fishing pole to support a 6.5m length of 10mm dia wire (The outer screen of some 75 ohm satellite coax). As I hadn’t got a long enough pole to support a full length element (7.0m) I had to wrap it around the fishing pole using about one turn every 0.5m, even using this technique I couldn’t quite get the full required length. The fishing pole was mounted on top of a 6m aluminium pole of 50mm dia. The earth connection at the input of the transformer was connected to the top of the aluminium pole, and the coax feeding the transformer was strapped to the side of the support pole.
Ignore the lines sloping down from the transformer; these are just thin guy ropes.
The next test was to measure the performance. For this is used a remotely controlled Icom PCR-1000 receiver and Datong active antenna located about 2 miles away from the TX site.
The following graph shows the performance of the antenna relative to a 7m vertical. This had been sited in exactly the same position and fed via a remote auto-tuner and 4:1 balun at the antenna base. Also for a further comparison I have included the results from a ZS6BKW (G5RV type) antenna which was mounted with its apex at the same height as the top of the vertical.
Note that the ZS6BKW gain can seem to be a lot lower than the verticals on some frequencies, due to nulls in the pattern towards the receive site. Also note that measurement errors of up to about +/- 2dB on each frequency have been observed due to slight changes in propagation conditions. The receive levels are measured in dBm so the higher up the graph the greater the gain. Each vertical grid line represents 6dB change ( 1 S-Point).
As can be seen from the graph the 6:1 transformer tuner-less multiband vertical works much better than I had predicted, in fact I think it’s quite impressive. So I decided to see if I could optimise the design and build a better version.
As part of this research I took another look at the reviews for the CHA-250 on eham, as I couldn't understand why some folks got good results with the antenna and others seemed to have really bad ones.
My measurements seemed to demonstrate moderately good performance, so I thought I would model an elevated feed in EZNEC.
It turns out that the support pole I used 6m long 50mm dia and standing on concrete (so partially insulated from earth) is almost ideal in terms of the radiated elevation angles. This seems to be because the whole antenna operates in a manner similar to centre fed vertical dipole.
Elevation plot showing antenna mounted on top of 6m metal mast 50mm dia. Base insulated from ground.
If you look at the plot below, showing the same antenna but with the bottom of the support pole connected to ground, you will notice that the angle of radiation steadily increases with frequency, resulting in a lot of the transmitted power being wasted.
Elevation plot showing antenna mounted on top of 6m metal mast 50mm dia. Base connected to ground.
So grounding the base of the mast seems to makes matters worse. I guess I just got lucky with the arrangement I used for my trials. This data would suggest that the length and type of support structure, plus decoupling of the coax at the correct length may have an important part to play in obtaining the best results.
Schematic diagram of the basic antenna configuration as suggested by Comet.
As further research I wrote emails to people who had posted bad reviews for the Comet CHA-250 on eham. The purpose of this was to try and determine if there were any common factors associated with the antenna installation which could affect the performance.
From the replies I received and further analysis of reviews I concluded the following:-
The main problem would seem to be the suggestion that the antenna will work with no radials, ground plane or counterpoise wires. This means that if none of these are installed, the antenna will use the coax cable as part of the radiating structure. This can result in very unpredictable results. I would strongly suggest that some form of ground plane is used. Adding a minimum of three sloping radials (at least 5m long) dramatically improves the performance, especially on 3.6 and 7MHz.
Many people had unrealistic expectations of antenna performance, especially on 3.6MHz where comparisons were made with dipoles, frequently for medium distance contacts. The antenna is electrically short on 3.6MHz so even if an auto-tuner is used the performance will still be 3-4dB down on a 1/4 wave vertical (20m high). The loss in the matching transformer at this frequency reduces the gain by a further 13dB, so the total gain at 3.6 MHz is about 17dB less than a 1/4 wave vertical. The other factor is the lack of NVIS coverage which makes a big difference for semi local contacts on 3.6MHz.
Other people made comparisons with vertical antennas such as the SteppIR BIG IR or Array Solutions Zero Five with these antennas the installation instructions give advice regarding good installation practice. There is nothing magical about these antennas. But if the installation instructions relating to earth radials, grounding, feed line routing etc. were applied to other vertical antennas perhaps more people would experience better results.
Another set of comparisons were made against antennas such as the Solarcon I-Max 2000 this is a 24 ft CB base station antenna which has a wideband matching arrangement permitting operation over the 18-29MHz frequency range. I estimate that my version of the Comet antenna has about 3dB less gain than the Solarcon at 29MHz, which may result in a loss of some contacts. This is particularly true on the higher frequency bands where interference and other background noise is low, making weak signal operation possible. This is not the case on the lower frequency bands in an urban environment where noise tends to mask very weak signals, so the received signal to noise ratio can appear to remain the same when using a lower gain antenna.
Shortly after making the last set of measurements I bought a SG-3000 auto-tuner. As a further experiment I decided to perform some additional tests. I set up a 7m vertical on a balcony and fed it with the auto-tuner, 6:1 transformer, and 4:1 balun and remote auto-tuner (the set-up I had used for pervious measurements). As an additional precaution I used several 1:1 isolating chokes on the 10m coax feed in order to ensure that the cable was not radiating. In addition to testing the antennas with a single 6m counterpoise, to simulate a metal support pole. I also tried each permutation with a selection of extra counterpoise wires in order to determine an average relative gain value for each arrangement. As before I used a remote receiver located about 2 miles away from the transmitter site.
The results were very revealing, as expected the 7m with CG-3000 auto-tuner at the base gave the best performance, so I used this as the reference level and compared the other configurations against it. I averaged the results with different counterpoises in order to give an overall figure of merit for each antenna.
From the graph below it is possible to see the general trend. The gain relative to the ATU at the base is about -6dB (one S point) at 7MHz and -3dB (1/2 an S point) at 29MHz. As would be expected the gain at 3.6MHz is low at around -9 to -12dB. (1/2 to 2 S points) At 51MHz the gain is likely to be slightly worse than shown, as the CG-3000 is not really rated for 50MHz operation. In fact the 4:1 balun and remote auto-tuner work better. If this is used as a reference then the 6:1 transformer has about 3dB less gain than shown on the graph at 50/51MHz.
So in most cases the 6:1 transformer gives very similar results to the 4:1 balun and remote-auto tuner. This was a real shock, as most experts recommend using a 4:1 transformer when a tuner is used remotely in order to reduce the matching loss from the coax under poor VSWR conditions. What I hadn't realised was just how much additional loss can be introduced when the secondary of the 4:1 balun is not matched correctly, or when baluns with Iron powder cores are used. See my general notes on balun construction and problems with Iron powder cores. This is effect is particularly noticeable at 1.9 and 3.6MHz where the base impedance of the 7m vertical is very low resulting in significant additional loss.
This made me wonder if it was possible to improve upon the balun design, in order to reduce the through losses and provide a better match to the impedance presented by the vertical element.
Based on this research I built a new design of transformer, raised the height of my 7m antenna so that the base is now about 6.5m above ground and added three 10m long sloping radials just below the transformer, which also double as guy wires. This has increased the average gain of the antenna by approximately 3dB at 3.6 and 7.1MHz and 1 to 2 dB over the remaining frequency range. It is now performing better than the original configuration using a 4:1 balun and remote auto-tuner. You can find construction details here
Several other people are now building similar versions including Giovanni IW2EN
In conclusion I would say that the antenna is capable of reasonable performance, especially if some form of counterpoise or radials are deployed. Apart from operation on 3.6MHz the performance is comparable to using the same height vertical with a 4:1 balun coax fed from a remote tuner. The only way to further improve the performance would be to use an auto-tuner at the base of the antenna, and add more radials.
If you don't wish to build your own version and are considering spending the amount of money an antenna such a the Comet CHA-250 costs (or any other multiband vertical antenna), I would suggest that vertical wire supported by a 10m fishing pole with something like a CG-3000 auto-tuner at the base, may be more cost effective and a better performer, especially on 3.6MHz.
