Because of many requests from international radio friends who tell me that translation of Google from Dutch to English (or other languages) is not that good, this page is in English. Hopefully, I can help / inspire somebody outside The Netherlands as well. I’m not that familiar with the imperial system, but if imperial measurements are easily calculated, I will do for you. 1″ is about 2,54 cm (multiply meters and divide by 2.54 to get measurements in inches) so you do the calculating yourself OK? Als je Nederland bent en je begrijpt dit artikel niet, stuur me dan gerust een emailtje of plaats een reactie op dit artikel gewoon in het Nederlands.
I was using a very short homebrew rotary dipole for 20/10 meters. It was only 2 x 2,6 meters wide making a normale 1/2 wave dipole for 10 meters. At the ends it had a coil that electrically “disconnected” the 1m long wire end after the coil. At 10m height (33 ft), the impedance on 28.5 MHz was around 78 Ohms. Optimal impedance on 14 MHz was close to 50 Ohms but bandwidth was very limited (less than 70 kHz) which is annoying when you’re using an amplifier and manual tuner like me. Performace on 10m was “okay” but no more than that and it is my belief that reception on 20m was worse than what I was used to (full size wire antenne). For 10m I will be building a Moxon that will share it’s boom with a 3 element yagi for 6m that is capacatively coupled by the 10m radiator (see other article on this antenne; only 2 x 1,9m wide and fits on a 2m boom). So, I no longer needed a rotary dipole for 10m and wanted to improve performance on 20m. For a long time, I tried to design a dual band dipole for 15m and 20m, but every design had something that I didn’t like. That’s why I gave up 15m (a band that I don’t have anything with to be honest) in favor of 20m. Hopefully, the 20m monoband design will tune up to 15m.
My dipole will have two 3m (= 10 ft) “legs” made of 30mm (1.2″) aluminium tubing. At the end of each leg, there’s top capacity; a cross made of 1m aluminum 10mm tubing. I will flatten the middle of two 1m long 10mm tubes and bolt those to the end of the 30mm leg so that it makes a cross of 4 x .5m (20″). The normal 10mm tubes that you can buy at the DIY stores (Gamma, Bouwmarkt etc.) will allow sliding in a 8mm tube that they sell as well. I will be using small stainless steel hose clamps to fix the 8mm tubes inside the 10mm tubes. This will cater for a length of 1m (3ft) to 1.5m (5ft) which is excellent because each element of the cross needs to around 1.2m (4ft). Tuning this antenna, will be done by tuning the capacity cross elements. It sounds fuzzy, but this is what it looks like (picture paints a thousand words):
So now you get the idea right? I will make the legs of two 3m length pipes, of course you can use 2 or 3 tapered pipes per leg, you figure that out for yourself. I’m not that good in construction, so a solid 3m length will ensure good electrical connection and fysical strength. I’m planning to use this antenna as a base antenna, so portability is not an issue here. Here are the dimensions that I will be using (copy of the input window from MMANA antenne modeling software):
I optimised the lengths for MY situation; 10m (33ft) above (moist) ground level. If you use different diameter tubing or want to set it up at a different height (13m or 40ft seems ideal according to MMANA; it will add 1.5 dB gain!), you might want to use MMANA to experiment with the correct lengths that work best for YOUR situation. If you don’t know how to use MMANA, just let me know you’re situation and I will probably do it for you, I’m kinda hooked to MMANA I think (hands start to tremble when I haven’t designed an antenna at least twice a day HI).
So how does it perform? Let’s see…
At 10m height (my situation), the maximum gain is at 29.4 degrees which is OK for allround radio fun. For DX you might want a lower angle but anywhere between 20 and 35 degrees is fine for me. Note that when you place it lower than 10m AGL, the angle will increase and the gain will decrease. At heights less than 5m, you probably have a typica “cloud burner” The max. forward gain is 6,75 (evil) dBi. It’s not that I want to brag with big numbers, MMANA simply doesn’t calculate dBd’s in “real ground” mode. I did a calculation in free space, the maximum gain is then -0.2 dBd; it other words, the difference in performance with a full-size dipole is very minimal. If you’d calculate the gain in real ground situation for a full-size dipole at 10m AGL, you’d find that it is around 7 dBi so yeah, my compact dipole performs like a full-size one. That’s not a miracle but just common sense; my antenna holds the full size length in materials, but it’s not erected as one 5m long element. As the current in the end of the antenna is minimal, the effeciency decrease caused by “folding back” the end of the antenna is minimal as well. No miracles or HF voodoo here. Then gain (attenuation) on the sides is -2.6 dBi. This means that rotating the antenna 90 degrees, will give reception a TOTAL maximum benefit 9.35 dB; the stations in front (and at the back) of the antenna are 9.35 dB louder than the ones on the side, hopefully certain (not all!!) South European stations HI.
As you can see the impedance is very close to 50 Ohms which is very nice because 50 Ohms is what the amplifier wants to see. More important is that it is 50 Ohms at almost the some frequency where the reactive resistance equals 0, in other words, it is not only 50 Ohms,it is in resonance as well which is much more important; impedance you can match, but resonance you can’t!! I’m really not a great technician and even I can understand that, I don’t understand why so many radioamateurs can’t. In MMANA, you don’t look for 50 Ohm impedance, you look for the point where Jx = 0 because that’s where the antenna is resonant. If it’s resonant AND the impedance is close to 50 Ohms, then you hit the jackpot because matching impedance means extra work, costs, losses etc. That’s why I like simple antenna design so much.
‘Nuff said… Here’s the 3D radiation pattern AND the VSWR chart (no that I know that it’s resonant, YES I like it to be as flat as possible I’ll admit):
Yeah that’s right; a resonant antenne for 14 – 14.350 MHz that’s only 2 x 3m wide and has VSWR better than 1:1.2 on full 20m band. No difficult to make lossy loading coils, just aluminium tuning. Simple, straight forward. Pretty cool duh?!.. If you want to make it even smaller; every cm (or inch) you add to the capacity cross elements you can take off the 3m legs. Be aware that your impedance will go under 50 Ohms. As long as it’s around 40 Ohms, you’ll be fine! Even though it’s not the design for me, you could make it 2x 2,5m wide and have your capacity cross elements 1.35m long. It will work if you accept a slightly higher VSWR. Experiment or let me know what you can put up. I bet it will even work with 2x 2m legs and 1.5 – 2m thin capacity cross elements.
Feed with a 1:1 balun or choke. Diamond makes a great BU-50 for only €30/$40 which saves you a lot of hassle. 1:1 baluns are not that difficult to make so you could make one yourself. A “lazy balun” (choke) would be some windings of your feedline coax on a Amidon FT-240-63 core or just use the simplest/cheapest (and according to my experience in practice: pretty good) method by coiling up a length of coax just below the feedpoint: around 6-8 windings with 10-15 cm (4-6 inch) will make a fine choke balun. While your (hmm.. MY) antenna is resonant and has 50 Ohm impedance, I don’t think there will be much “shield current” flowing back to the shack anyway but let’s just put in some unbalance to balance feature just to make it a bit more scientific HI.
Friends I don’t hold the license for this design and happily share it with everbody. I have learned (and still learn) so many things from all the elmers out there that I’m glad that I can make a small contribution to the ham radio community. If you want to call this dipole the HHdipOle (my call is PA3HHO) I would be honored. The PV-dipole would be fine to (my initials) but if you want to name it “[your call] dipole” that’s fine with me as well. End capacity dipole antennas are common practice, I just put a little effort in using the theory in practice. I hope you enjoyed this article. If you did, let me know by replying to this article. I would love to see your versions. I will publish pictures of mine very soon (still building).
73 es good DX on 20m!
Pleun Vermeulen – PA3HHO