Making Waves – A DIY HF Antenna for the smallest spaces
Is your garden the size of a postage stamp? Or maybe you live in a flat/apartment with just a balcony, then this could be the antenna for you to get on the air using HF frequencies. We all know that an HF dipole for 40m (7MHz) is going to be 20m (66ft) in length and not everyone has the available space to fit one in (hell, a 1/4 wave vertical is going to be 10m (33ft) long and may require planning from your local authority. So how about a vertical antenna that is only 1.7m in length and covers all HF amateur radio frequencies from 7MHz to 28MHz via your ATU? Impossible I hear you say, not at all – read on to see how it is done….
What will you need? Not much really, you will need a length of plastic pipe, 1.70m long, 41mm in diameter – this is sold in the local hardware stores as 40mm waste pipe but be careful, 40mm is the inside diameter and it is available in either 41mm or 43mm outside diameter – 13.6m of 1.5mm insulated copper wire, some 20mm long cable ties (200 * 3.6) and a 4:1 UNUN (see later for instructions).
Effectively, take One end of your wire and secure it to one end of the 1.7m pipe using one of the cable ties. Wind on 67 turns of the wire over a length of 220mm and secure with a cable tie (insulating tape can also be used). Now drop the wire vertically for a length of 360mm and secure again. Wind on 22 turns (in the same direction as previously) over a length of 70mm and secure again. Then wind three turns, wide spaced over 820mm (and secure again leaving the end free. This end then needs attaching to your 4:1 UNUN.
For the 4:1 UNUN you can either build your own by following the image below:
Or buy one ready made (Magituner A) from www.m0cvoantennas.com .
Leave a short amount of space at the bottom of the pipe to fix either a bracket (if you are going to pole mount it) or use cable ties to fix it to railings on your balcony, etc. You will need to attach an earth wire to the second peg/machine screw. This only needs to be short – maybe 4 or five foot but does help with the matching. Attach coax to the SO239 socket using a PL259 plug and run back to your radio via an ATU (the internal one (if fitted) may be able to cope with this) tune up and away you go.
m0cvoantennas 
can anyone tell me about the 1,5 mm copper wire. I assume this is solid core wire with enamel coating like wire in a transformer or a motor. Not just wire with a plastic jacket.
Could someone clarify this point for me. Thanks Ken VE1PDL
Hi Ken,
It is stranded, insulated copper wire – type ST91X. This is the same type as used in many antennas – dipoles, etc.
Hello I put your antenna together, am I supposed to have continuity between the center pin and ground on my coax to the transmitter? The unun diagram shows the other end of the ground connected to the pin. I am a new general and trying to get on the air for the first time in the US with very little space for antennas.
Hi Joshua,
If you measure across the center pin and the ground of the socket with a DC multimeter it will show a short. This is common with what is effectively an AC transformer. Also if measured directly with an antenna analyser or VSWR meter it will show a poor SWR. The whole thing is based primarily on the Rybakov Antenna (http://www.learnmorsecode.com/iv3sbe/rybakovantenna.html) but the helical system for this particular one I designed a few years back and marketed as the HFC3015-V but this is no longer in production as the overall size (1.7m) made shipping very expensive and most carriers wouldn’t take it.
I built the antenna and so far, I only hooked it up to external antenna tuner and antenna analyzer but I am optimistic it will work from my 1st floor apartment patio. SWR looks great 30m to 12m. SWR was about 2.8 on 40m but I am hoping my internal radio tuner will compensate.
One construction “issue” – I cut my 16AWG wire to 13.6m = 44′-7.5″ and wound it on 1-1/4″ PVC (OD=42.5mm). All of my windings matched the design lengths, EXCEPT the final 3 turns. The final 3 turns would only run for 11″ (about 275mm) – I could not “stretch” the windings to the design of 820mm. Any thoughts?
Roy K2RMA
Hi Roy,
It is down to the outside diameter of the PVC pipe. I used 41mm OD pipe – here it is available in either 41mm or 43mm OD. The 1.5mm extra diameter when multiplied by the number of turns does make up a fair length which means that the final bit would be shorter. Shouldn’t make too much difference though. The SWR of 2.8 on 40m is to be expected as the wire length (13.6m) is a full wavelength at 21.200MHz (15m) and a 1/2 wave at 10.105MHz (30m) – hence the original was the HFC-3015V antenna.
Nigel M0CVO
Nigel, thank you for your quick reply and helping me understand my wire length mystery. Geez, just 1.5mm difference. I will report back on performance when I hook up my radio this week.
Well, initial testing show it’s working! From my new QTH in Shelby NC, using my ICOM 756 Pro3 (with built in tuner) at 15 watts, I QSO’d on FT8:
17m Idaho, California
30m Florida, Bulgaria
I need to clean up my installation, etc. But I am quite happy with my working “apartment” antenna.
de Roy K2RMA
Glad to hear it is working well for you Roy. It may be a compromise but it does the job.
73 DE M0CVO
Hi Roy,
The extra length created by the 1.5mm extra diameter is ~138mm or 5.43inches.of course, there are also other factors to consider like the thickness of the insulation, any gaps between the windings, etc.
Will look forward to your performance reports.
73 Nigel M0CVO
Nigel, I just broke out my slide rule and also calculated for my “missing” length . Now I just need to unpack and set up my ICOM IC-756 Pro3. So my actual pipe length is 3.5 feet rather than expected 5.5 feet. Does anyone need or want to buy 2 feet of 1-1/4″ PVC?
Hello Nigel, thanks for posting this antenna. I made it using 13amp mains cable from a Dyson Vacuum LOL. Brown and blue wires soldered together and shrink tubed at the joint. Once made I attached my home brewed 4:1 air cored balun and it wouldn’t tune every band using my Yaesu FT 450. I tried a few different lengths counterpoise and all did, but not at one certain length ? I waited until this morning and placed outside my flat where I have a field of various radials under my Council lawn and all bands tuned no issue ! I tried FT8 on 15m 10 watts and straight away had a -10 report from Germany. This with the antenna at ground level, so the 4:1 balun at ground level. I did notice noise was quite high and I did get some RF on the coax, but will add a choke balun and retry. Is there a height I should ideally put the antenna at ? I look forward to you reply and trying this antenna out some more on various bands. Thanks again Andy M6APJ
Hi Andrew,
Your FT-450 probably wouldn’t tune it because you effectively made a different antenna. This antenna uses a 4:1 UNUN, not a 4:1 BALUN. The antenna itself is an unbalanced vertical hence the use of an UNUN – UNbalanced antenna to UNbalanced feeder (coax is unbalanced). A BALUN is for use with a balanced antenna such as a dipole or loop.
Nigel M0CVO
Nigel, Running FT8, it seems like I cannot adjust my rig power on 40 and 20 meters – it stays “stuck” at 10 watts. I can adjust power on 30m, 17m, 15m, 10m a lot higher than 10 watts (15-25 watts easily). I am using my rig ICOM IC-756Pro3 tuner. I tried a counterpoise at 2 different length (about 5 foot and 10 foot) but no difference. Any clues on what I should check or change?
Thanks, Roy K2RMA
Hi Roy,
If it’s anything like my IC-746, I know the rig’s logic system will automatically reduce the power when presented with a constant carrier mode such as AM, RTTY and various USB digital modes (FT-x, PSK, etc). It will also reduce/limit the power when presented with a SWR of >3. As mentioned previously, the antenna wire is cut to a length that is a full wave at 15m and 1/2 wave at 30m so possibly 40 and 20 are the bands that present the highest mismatch for the tuner to cope with and so the rig limits the power. Have you tried it on other modes (CW/SSB) and do you get the same results?
(I guess I should know this but not sure…..) Would changing the radial length reduce the SWR? What is your recommendation for radial length? Do they just hang from the unun towards the ground?
So I am a bit stymied. I am still trying to troubleshoot my issues on 40m and 20m. My configuration has high SWR on 20m and 40m – so my rig is not transmitting. Experimenting, I found that if I increase my 30 foot coax length by 10+ feet, my tuner now gave SWR around 1.0 on 20m and 40m. Using PSKReport, I am able to see that my 40m transmissions are seen. But my 20m transmissions are never seen. I tried changing counter-poise lengths, etc but it seems to make no difference. Any suggestions for troubleshoot? Note that 30m/17m/15m/12m/10m seem to be working. Thanks, Roy
Hi Roy,
If changing the length of the coax changes the SWR then the UNUN is using the coax outer as the earth. Changing radial lengths may help with this. Winding a choke (ugly balun) in your coax may also help.
So I am building your antenna and after reading some of the comments I thought I should get it straight in my head. Ok, you say an earth ground.. are we talking a ground rod and attach it there or a bunch of counter poise legs 4or 5 feet long? I am enjoyin’ the build so far Nigel.
Hi Bill,
It really depends on your needs and available situation. I used a couple of wires at 4 foot in length but, if you are mounting it above a balcony you could simply attach a single wire to the balcony rail or, if low in the yard to a ground rod.
Nigel, M0CVO
Hi,
Thanks for sharing this antenna. I plan to build it. Do you have any infotmation about the transmitting power it can sustain?
Hi Peter,
If made with the materials as described (including the T200-2 core for the 4:1 UNUN) it will handle 400W of CW or SSB with ease. Not sure about FT-8 or similar constant carrier data modes though. These will be less.
73 M0CVO
Hi.If you were to wind an Air 4:1 UNUN , using the pvc tube under the windings as a former, rather than a ferrite UNUN , do you think that would work ?
Hi Rod,
Not something I have tried to be honest but it may work – experiment, it’s what the hobby is all about.
73 M0CVO
Hi Nigel
I decided to have a go at building your antenna as i dont have a lot of space for antenna’s and only recently come back to radio after 20+ years away i’m pleased to say it works have dot a vswr of 1-1.3at the centre of the15m band and 1- 2.8 on 40m the atu brings it down nicely to 1-1 had 2 contacts with it on a xiegu qrp set (5watts) Belfast and spain am now going to try it with ground plane radials will let you know with further testing……
john m1alg
Hi John,
Sounds like it’s doing the job then. It should also match well on the 30m band, especially once you have fitted the radials. Welcome back to the hobby.
73 DE M0CVO
oh by the way its 10 ft away from me at the back of the shack
Nigel, I built the antenna as described and used 3 20 foot radials. Great RX but I couldn’t talk to anybody. I moved it to the other side of the house and connected it to my radial field that I use for a vertical and made 3 FT8 calls. Everybody says it needs a radiator which I haven’t tried, but may be a good idea. It tunes uo on all bands but doesn’t get out. Any thoughts? 73
Hi Jim,
It should work fine over the three 20ft radials. The antenna itself is the radiator so not sure what people are talking about.
I fitted mine to a short fibre glass mast (about 8ft) and managed contacts across Europe with it using a single radial of 4m (13ft).
Nigel.
Hi Nigel,
Is there a way to have this antenna work on 80 meters by probably adding to the length of the antenna on the top part?
I like the short length. Is there an easy way to “lengthen” the antenna to say about 4 meters to have a lower take off angle, and at the same time cover 80 to 6 meters bands?
73s
Ed
Hi Ed,
I have not tried it myself but you could probably get it to work from 80m by doubling up on the lengths. Of course, an half wave at 3.5MHz is ~40m but with the induction in the coils this could probably be reduced to about 30m for the wire length. Tuning may be tight though but it is worth experimenting. If you do try it, please let me know how you get on.
73 Nigel, M0CVO