Radiation Coupled Dipole (RC-Dipole)

I needed an 80m antenna that fit into the available space of our suburban block. However living in suburbia and having space and sky-hook issues the task seemed to be daunting. An 80m ½ wave dipole has a span of approx. 40m, which is to long for most suburban blocks and certainly for our block. Since I also have sky-hook problems, it would be good if I could use the antenna on multiple HAM-Bands. But I don't want to use traps. Not that there is anything wrong with traps, but I think building good low loss traps is a lot harder than building a good loading coil. I only have approx 30m between my two sky-hooks and I would like the antenna to be feed with a single 50 Ω cable. So to wrap it up, my requirements are:

• • • • • • • • • length of the antenna approx 30m

                • 50 Ω feed-point

                • min. VSWR 2:1 bandwidth 100 kHz on 80m (centered at 3.6 MHz)

                • min. VSWR 2:1 bandwidth of 200 kHz on 40m (centered at 7.1 MHz)

                • min. VSWR 2:1 bandwidth of 50 kHz on 30m or

                • min. VSWR 2:1 bandwidth of 200 kHz on 20m (centered at 14.15 MHz)

                • no traps,

                • no special matching network,

                • good efficiency.

Studying Antenna-books [1] and information found on the Internet [2], I decided to use a length of ⅜ λ for 80 m as the foundation for the antenna. Using a program I wrote to calculate loaded antennas, I calculated the inductance of the loading coil and the best position for the loading coil. (The program calculates one half of the antenna only)

Here is the output of the program:

With this information at hand it was easy to design the first part of the dipole. The part between the loading coils is 26.2 m and each loading coil has a 2.5 m pig-tail for a total of 31.2 m. Well, a pretty good start.

Here are the dimensions of the 80 m loaded dipole.

Lets plug this info into the computer and model the antenna [3] to see if it will perform at my QTH. No good to model the antenna in free-space for obvious reasons, so I modelled the antenna at 7 m height over poor ground, to reflect installation at my QTH.

So lets see what the model has to say.

Here are the initial results of the first run again.

Not a bad first run, but room for improvement. A quick run to optimize the VSWR and here is the final output of the designed antenna.

This turned out to be rather nice and easy. An impedance of 65.5 -j0.66 and the a 50 Ω VSWR of 1.31:1. But what about the VSWR bandwidth? Lets look at this graph.

This doesn’t look to bad.

Now to the performance of the antenna? Well, to check how well the antenna performs, I modelled an 80 m full-size dipole at the same height (7 m) and with the same ground parameters (poor).

Here are the modelled results of the 80 m dipole.

and a VSWR sweep over 3.5 to 3.8MHz.

As can be seen from the plot, the 2:1 VSWR bandwidth is approximately 160 kHz, twice that of the designed loaded dipole. So this is what I'm going to use as the baseline to which I will compare the loaded dipole.

Now lets see if the loaded dipole is a dummy-load.

About 1.5 dB difference, nothing to worry about. However, the antenna has limited bandwidth of approximately 80 kHz at a 2:1 VSWR but it fits nearly into my available space.

Next on the list of requirements is 40 m coverage. But I don't want to use traps. I could add additional dipoles directly to the feed-point, but from experience (Hello Al) that causes tuning nightmares and interactions. I remembered seeing a HyGain Explorer 14 using additional elements to increase bandwidth on 10 m by using elements of different sizes. But also some of the VHF/UHF ¼ λ vertical antennas are using this principle to make the antenna work on VHF and UHF. So I gave it a try and the result is the RC-Dipole.

The antenna would look like something like this:

The bottom element is our base antenna for 80 m, the loaded 80 m dipole and above we have a 40 m ½ λ element. This element can be above, below, in front or behind the main element.

Back to the modelling program and after a few trial and error runs, I eventually found the right mix of spacing between the main element and the radiation coupled (RC) element. No changes had to made to the main element.

Here are the dimensions of the 80/40 m RC-Dipole.

These are the modelled results for the 40 m radiation coupled dipole element (RC-element).

and below the modelled 40 m VSWR sweep.

This looks pretty good, with a 2:1 VSWR bandwidth greater then 200 kHz. And since most of my operation is between 7.0 and 7.2 MHz, I'm happy with the results. Now lets see if we have changed the good performance of the main antenna element.

Here are the modelled 80 m results with the 40 m RC element added.

Well, isn't that interesting. The modelled result shows an even better VSWR.

So lets look at the performance of the antenna.

Would you look at this, what a pleasant surprise. The plot shows us that the RC-Dipole's gain has increased more then 1 dB. There's still a gain difference in favour of the fullsize dipole, but the difference is small, less then 0.5 dB. The increase in gain could have something to do with the increase in antenna aperture and the current distribution in the added element (or the modelling program can not handle closed spaced elements correctly) so lets not get carried away with the increase in gain. I've also run a VSWR sweep of the antenna from 3.5 to 3.8 MHz and I can report that the 2:1 VSWR bandwidth has increased to 100 kHz.

Lets take another break and check against my requirements.

• • • • • • I now have a short 80 m dipole with a 100 kHz 2:1 VSWR bandwidth (32 m vs 42 m)

          • I am able to transmit on 40 m with a bandwidth of more more than 200 kHz 2:1 VSWR bandwidth.

          • The performance of the antenna on 80 m is nearly as good as a full-size 80 m dipole. (< 1 dB diff)

          • I have a single 50Ω feed-line cable.

          • And I have avoided using traps.

Well, I'm nearly there. However the length of the antenna is still an issue, approximately 32 m vs the required 30 m, and I also would like the antenna to work on either the 30 m band or the 20 m band, with 30 m the preferred option.

Well to cut a long story short, it looks like I ran out of luck. I could not get a reasonable 50Ω VSWR on 30 m, which means that 30 m is out.

So, lets see what the model brings us with an additional 20 m RC-Element.

If we look closely at the centre of the 80 m loaded dipole (13.1 m) we should note that the length is very close to be ⅝ λ on 20 m (13.3 m). I had the suspicion that the antenna will not be easy to model with these dimension on 20 m unless I extend or shorten the centre part of the main antenna. Sure enough, a quick run of the model proved that I had to modify the base element of the 80 m dipole.

Since I knew that I'm only out by a few centimetre (20 cm) for a ⅝ λ at 14.1 MHz I've lengthened the centre part of the main, the 80 m element, by 0.2 m to 13.3 m which brings the centre part of the antenna to a ⅝ λ at 14.1 MHz. I then readjusted the 80 m pig-tails and checked the 40 m element and voila, success, the 80/40/20 m RC-Dipole has seen the light.

Below are the 20 m results of the 80/40/20 m RC-Dipole,

and next the 20 m VSWR sweep of the same antenna.

I've then run a few checks against the model on 80 m and 40 m to see if there are any changes to these two bands, but I can report that the changes are that minute (mm) that they are not worth worrying about.

So, here are now the dimension for the 80/40/20 m RC-Dipole antenna.

If you would like to build the antenna checkout "Building the 80-40-20m RC-Dipole".

Well, going over my requirements I would say 90% (87.5%) achieved, if only I had more than 30 m to hang the antenna up. But, before I get to that dilemma, lets wrap this model up and have a look at the radiation pattern at 7 m above and over poor ground for the 3 bands, 80-, 40- and 20 m (3.6-, 7.1- and 14.15 MHz).

Well, no surprise here. The antenna is to low so most of the radiation is straight into the sky (NVIS). But it is a rather clean and for my requirements acceptable pattern.

Again, as has been seen on the 80 m pattern the strongest radiation is into the sky. However, it can already be seen that there is a nice bit of gain at the lower angels which might allow some DX on 40 m with better than average condx.

Modelling the antenna with the pig-tails pointing down showed that the resonance frequency on 80 m had moved from 3.6 MHz to 3.64 MHz. (400 kHz) A bit to much of a change for me, but easy to rectify. Adjusting the length of the pig-tails and the antenna was back at 3.6 MHz. But what about performance? Had I lost anything?

Well, here is a comparison gain plot between the straight tails and the dropping down tails.

OK, there is a less than 0.5 dB difference in gain when dropping the pig-tails down. Not bad, not bad at all. A quick run of the model to check the rest of the antenna showed no changes on 40- nor on the 20 m band. Even the 80 m VSWR 2:1 bandwidth had not changed.

Well that's it. I've managed to fulfil all my requirements.

Here are the 80 m and 40 m VSWR plots at the end of some 25 m RG-213 (no BALUN).

As can be seen the environment and feeder play a role when modelling antennas, but we do get rather close to the initial values. With a little bit of pigtail tuning we'll be able to tune the antenna for a 100 kHz 2:1 VSWR between 3.5 and 3.7 MHz to suit.

And, how did the antenna work for me?

Well, the reason for the antenna to come to life was that I wanted to partake in the Australian Remembrance Day Contest (RD-Contest). However, with the current band conditions as they are, and having only antennas up to the 40 m band, I couldn't see me improve on my previous scores.

Going through the 80 m logs indicates that the antenna has performed as expected, good to very good coverage across the 2000 to 3000 km range. I've been able to work every station that I could hear! I've worked all over VK including stations from VK6 (approx 3000 km) and ZL (approx. 2200 km) with very good signals reports.

The performance of the Dipole on 40 m is good and having a K3 with the second RX, enabled me to use diversity reception with my multi-band vertical (Cushcraft R8). A big plus with the current unstable condx on 40 m. As I have only build the antenna for 80 and 40 m I'm unable to comment on the 20 m performance (yet), but would expect similar performance as the 40 m part of the antenna.

I would hesitate a guess that if you have similar restrictions, i.e. limited number of sky-hooks, height restrictions, small block, you will find that this antenna will get you on the air with good performance on all three bands. Performance will, of course, improve with height even lifting the antenna by three meters to 10 m will show markedly improvements in performance.