There are various methods known if you need to shorten a dipole while keeping it operational. You can use coil loading, linear loading, or you can add capacitance hats at the ends of the dipole arms. Usually, the length of a shortened dipole is around 1/4 wavelength. I wondered if it was possible to shorten it further to 1/8 wavelength while still preserving acceptable gain and bandwidth.
I modeled a number of shortened dipoles 1/8 wavelength long with various capacitance hats at their ends. The antennas described below are designed for 20 m band (center frequency Fc=14.175 MHz). You can rescale them up or down for other bands.
All models had a 2.5 m long main element with a feed point in its center. 2.5 m is about 1/8 wavelength in 20 m band. The mini dipoles differed in the type of hats connected to their ends. The majority of hats (but not all) were designed with cross shaped spreaders in mind. So, the supporting structure in most cases would look like in the sketch below.
You can find the views and performance parameters of the antennas I simulated in the tables below.
In most cases, there is only a small penalty in gain w.r.t. full length half-wave dipole (-0.7 … -0.5 dBd). The gain remains almost constant over the whole band (within +/- 0.1 dBi).
The radiation pattern of a mini dipole is practically identical with the half-wave dipole. Take a look at the radiation pattern shown below.
All models have resonance resistance close to 12.5 ohms, what make them relatively easy to match to 50 ohm coaxial cable. 1:4 balun like that in the G3TXQ Cobweb can be used for the purpose.
On the other hand, there are significant differences in size and bandwidth among the models. Probably the most popular shapes are the Checkbox and 1-Arm Spiral. The first has quite wide bandwidth, but the latter is a rather poor solution. You can notice improvement with the 2-Arm Spiral, but the 4- Arm Spiral is even better. I would not recommend building the 1-Arm Spiral version.
Triangle No. 1 and No. 2 can be of interest for those who would like to hide their mini dipole under a roof.
The mini dipole with Triangle No. 2 hats can be built on two vertical poles assuming 4 strings are added to anchor the lower ends of wires (it will look like a tent).
Click here to download the mini dipole models.
I also simulated shorter dipoles - 1/12 wavelength long. Their resonance resistance dropped to about 5.6 ohm, what is 1/9 of 50 ohms. The simulation results are shown in the table below.
1/12 wavelength dipoles have narrower bandwidth. Some hats are no longer practical due to too narrow bandwidth. The hats that still can be used (like the Checkbox) are rather big when compared with the antenna length. In my opinion, 1/12 wavelength dipoles are not practical.
Generally, when you shorten a full-size dipole by adding capacitance hats at its ends to maintain resonance at the same frequency, the following changes happen:
1) resonance resistance decreases,
2) size of the hats increases,
3) bandwidth decreases,
4) gain decreases.
1/8 wavelength mini dipole seems to be a good choice for a shortened dipole because:
1) resonance resistance is close to ¼ x 50 ohms, what makes it easy to match to a coaxial cable,
2) the size of the hats is still in acceptable proportion to the antenna length,
3) decrease in gain is small,
4) if the hat type is properly chosen, bandwidth is sufficient for the 40 - 12 m bands; half of the 10 m band or slightly more can be covered; about 100 kHz of the 80 m band can be covered although due to hat dimensions, the 1/8 wavelength dipole would most likely be considered by most of us as not practical for this band.
It is possible to build a folded version of the mini dipole. The folded mini dipole with 4-Arm Spiral hats looks like the drawing below.
The dimensions are the same as for the regular version: 2.5 m between the hats, and every hat is based on four 1 m long spreaders. The resonance resistance is 4 times greater and equals ca. 50 ohms.
The performance of the folded mini dipole is shown in the table below.
Radiation pattern is the same as for the regular dipole.
Current distribution is intriguing.
The current stays almost constant at every point of the loop. Only in the spiral arms not included in the loop, it decreases down to zero at their ends.
The space between the loose ends and the neighboring wires is about 10 cm. I think this is enough for transmitters outputting up to 200 W.
If you generate SWR plot of the above folded mini dipole in a wider frequency range, you will notice that it has harmonic resonance near 30 MHz.
Can you shift the harmonic resonance down a bit and get a 2-band antenna? Yes, you can. It is not difficult.
To do that, I added two drop wires: 1 meter each.
By this simple trick, I managed to shift the harmonic resonance down to make it match 10 m band while almost not affecting 20 m band. See the graphs below.
The radiation pattern on 10 m band is dipole-like but with some preference for the X direction at the cost of the Z direction.
And I think it can be interesting to take a look at the current distribution in this band.
The 4-Arm Spiral is not the only hat type you can use in the folded mini dipole. You can successfully use the 2-Arm Spiral. Perhaps there are more ways to do that, but I designed something like that:
The "tuning" wires run along the boom. And the basic performance parameters are shown below.
And the current distribution.
I have chosen the folded version with 4-Arm Spiral hats with drop wires to build and test.
That's how the experimental antenna looked like during the construction and testing.
Before building the experimental antenna, I modeled it in the second simulator - the MMANA-GAL. The M-G calculated the antenna resonances to be higher than in the 4nec2 simulator. I had to use longer last wires of the loose spiral arms to bring the resonances to the same frequencies as I had in the 4nec2 simulator. In the 4nec2 these wires were 60 cm long and in the MMANA-GAL - 71 cm. In the real antenna, I made them 75 cm long - just in case.
I hoisted the antenna 4.5 m above the ground. I did not dare to lift it higher because I used no guying to my delicate telescopic mast. The first SWR plot showed that the antenna was tuning too low. So evidently, the 4nec2 gave more credible prediction. I shorted the end wires by 12 cm. The measured SWR looked like that:
As you can see, I shifted SWR minima to the beginnings of the bands. If I had wanted to really use this antenna, I should have trimmed it further. Another 5 to 8 cm should have been cut off. However, I considered those results as sufficient to prove that the real mini dipole follows its theoretical model. And that NEC-2 engine in the 4nec2 did a better job than MININEC engine in the MMANA-GAL. In the EZNEC, you should expect the same results as in the 4nec2 - quite close to reality.
The real SWR is lower than simulated because of the impact of transmission line. I used 35 meters of RG-58. This is a rather lossy cable, so the SWR measured at the TRX end is lower than that at the antenna end of the cable.
I connected the folded mini dipole to my TRX and checked it against the Cat Whiskers broadband antenna mounted on a 10 m tall mast. During reception in 20 m band, it was only a split S below the Whiskers, and sometimes I could not tell which antenna produces stronger signal. The antennas were not ideally aligned for the test, so that was something to be expected. Also, the noise background level was very similar.
In the 10 m band, the folded mini dipole produced weaker signal than the Cat Whiskers. It sounded as if I had switched off one RF pre-amplifier in my TRX. The same was true for the noise background level. I think that was due to the low height of the test antenna. It could have been caused by larger signal attenuation of the RG-58 coax on this band (the Cat Whiskers were fed with a low loss RF-7 cable) or maybe the reason was due to different radiation patterns of the antennas mounted at different heights.
To sum it up.
You can build a folded mini dipole with 4-Arm Spiral hats and drop wires and use the antenna in two bands at F and 2F frequencies. The experimental antenna achieved enough bandwidth in both bands.
This experiment proved that the computer models of the antenna predicted reality quite well. You can trust the 4nec2 or the EZNEC simulation results. The MMANA-GAL predicted resonance to be on higher frequency than in the real antenna. But the difference was not very big.
Do not follow me on the materials I chose for the antenna. I never planned to use my experimental antenna for anything more than verification of the computer models. The PVC is too flexible, and it is not resistant to UV radiation. Maybe you can use PVC pipes for an indoor antenna, but rather not for the outdoor use.