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Build Yourself a 40/80m Antenna For Small Gardens Part 2.

January 14, 2019 3 comments

After writing the previous article on the small antenna for 40 and 80m HF Bands I was asked by several people for photographs of the antenna in question.  Therefore I have done this article as a follow on with pictures.

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Firstly you need the coil to be wound.  This one is 78 turns for 1mm ECW on a 41mm diameter former (plastic pipe) at 140mm long.  The screws hold the wire in place as described in previous blog post.

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Then you take two lengths of wire – one at 10.14m and one at 2m (or longer – I cut this one at 2.5m) and attach them either side of the coil.  Tape the longer section to a fibre glass fishing pole using insulation tape and allow the other end to lay loose beside it.  raise the pole and tie the hanging end off to somewhere convenient – I used the fence.

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A better picture of the coil on final position.

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Use of choc block to attach coax cable – second wire is a 10m long earth wire.

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The earth wire is just run around the garden.

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Final test using an AA30 antenna analyser shows the SWR at 3550kHz – just in the right place for a spot of CW DXing. Obviously , if you wanted the sweet spot higher, use a shorter length of wire at the end.

Build yourself a 40/80m antenna for small gardens

So, you’ve just got yourself a licence or a nice new HF transceiver and are hoping to be active on the lower frequency bands of 80m (3.5MHz) and 40m (7MHz) but are a bit concerned about how much space you will need for an antenna.  After all, an 80m dipole is going to be around 40m (132Ft) long and modern postage stamp sized gardens just can’t accommodate this.  So I am going to describe a single antenna for both bands that should fit into most gardens.

What will you need?  Well, firstly, you will need approximately 40m of insulated copper wire – the sort used for lighting circuits (single core, stranded), usually available from your local DIY or hardware store.  Also you will need a reel of 1mm enamel coated wire (ECW)this is available from RS Electronics – part number 357-788 – some choc blox (cable connectors) or powerpole connectors (available from www.sotabeams.co.uk), a fibre glass telescopic fishing pole – 10m would be ideal but anything from 7m up should suffice, a piece of 40mm drainage pipe (this will be either 41mm or 43mm outside diameter), some 4mm * 20mm machine screws and nuts, some 4mm ring crimps, a length of fishing line (fairly strong stuff 20kg strain or more), cable ties and a length of angled aluminium, or brass – also available from you local DIY or hardware store.

Directions:

Firstly cut two lengths of the insulated wire – one at 10.14m and the other at 2m. Strip a few mm of insulation from each end of the longest piece and from one end of the shorter piece.  Fit crimps where you have stripped insulation on one end of the longer wire, leaving the other end bare (you may wish to tin the bare end with solder) and on the stripped end of the short wire and solder them on for a stronger fitting.  These are going to make up your radiating element.  Put them to one side and we shall come back to them later.

Now saw a 140mm length of the 40mm pipe.  This is going to act as a coil former.  Measure in 1cm (10mm) from each end and drill a 4mm hole. Now rotate the tube by 90 degrees and drill another hole 10mm in from each end.  Scrape the enamel off the end of the 1mm ECW and thread it through the 1st hole on the left hand side of the piece of pipe.  Fit a crimp connector to this end and solder it for a stronger (and electrically better) connection.  Locate the ring beneath the second hole on the left side and push one 4*20mm screw through both and fix with a nut.  This now securely anchors the wire at one end.  Now wind 78 turns of wire onto the coil, you may wish to use insulating tape to hold the wire whilst you are winding it and once you have wound the 78 turns on.  At the other end, cut the ire long so there is plenty spare and thread it through the 1st hole at the right hand side.  On the inside of the pipe measure the wire to the second hole and make a notch/fold here.  Pull the wire out straight, cut it and scrape the enamel off.  Fit a crimp and again line the ring up with the hole and push a screw through, fixing with a nut.  This coil will act as an inductive load for 80m and a trap for 40m.  You may wish to wrap the whole coil in insulating tape to protect it from the elements.

Now take the 10.14m length of insulated copper wire and cable tie it along the length of the fishing pole with the bare end at the bottom.  Make sure the pole is fibre glass and not carbon fibre or graphite as these will affect the tuning of the antenna.  Start about 1Ft (300mm) up from the bottom of the pole.  The other end of the wire will be beyond the top of the pole, don’t worry about this, it needs to be.  Now at the far end of the wire, fit the crimp over the top of the screw you anchored the first end of the ECW to when you started winding the coil.  Fit another nut to secure it (you may fit an extra locking nut if you wish).   Now fit the shorter length of wire to the other end of the coil in the same manner.  Loop the end of the wire back on itself (about an inch) and secure with cable ties. Thread one end of the fishing line through this and tie off well.

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Schematic of wire antenna.

 Dig a small hole in the ground (the depth should be half the length of the piece of angled aluminium/brass that you have) and stand the piece of angled aluminium/brass in it.  Fill the hole back in and check that it is secure.  Raise the fibre glass pole to vertical and secure it at the bottom using cable ties (or whatever) to the portion of aluminium/brass that is protruding from the ground.  Using the fishing line that you fitted to the top end of the wire, pull it out to an angle between 30 and 40 degrees and tie the other end off to a fence or something secure.  You now have the basis of your antenna and I shall now describe how to feed the antenna with RF and get on the air.

Fit a choc block (or powerpole connectors) to the end of your coax – it will need to be a double connector for both the inner and earth/braid.  Then connect the inner side to the wire of the antenna and add one or two 10m counterpoise wires to the other side of the outer.  If you have space lay these out at right angles to the vertical antenna, if not, don’t worry, they can be laid in spirals or bent to fit in, it is the electrical length that is important.  You could also connect a short wire to a copper ground spike and lay shorter radial wires out from this.  Take the other end of your coax back to the shack and fit a PL259 plug.  This will attach to your ATU or radio.  Check tuning on low power (5W or less) – it should be resonant on 40m and 15m (3rd harmonic) and a small portion of 80m (remember it is loaded for 80m so possibly won’t cover the whole band.  You can adjust the length of the end wire to match it where required, or use your ATU.  Make sure you insulate the choc block/powerpole connectors to protect from water ingress.

You now have a 40m vertical antenna (as far as the inductor) and an 80m inverted L.  This will give you a near omnidirectional (all round) radiation pattern with a low angle take off so will be good for both nearby QSOs (depending on propogation and atmospherics) and low angle DX. Have fun.

73 DE M0CVO

 

SOTA – End Fed Long Wire Antenna Results

December 23, 2018 Leave a comment

Making Waves – 2m Quad

October 16, 2017 Leave a comment

The M0CVO High Gain 4 Element Quad for 2M

The antenna I am now going to describe is one that I designed some time ago. It is a high gain quad beam for 2M (144 – 146 MHz) band. The forward gain of such an antenna is approximately 11.5 to 12dBd, that’s approximately 10.6 to 10.8 times the output power from the rear of your transmitter. For example, say you were operating a 10 Watt txr, the effective radiated power (erp) would be 10*10.6=106 Watts.

All this power and still a relatively small antenna; the boom is a mere 1 metre in length and may be constructed from 1” (2.5cm) square, weather treated, wood. The elements are constructed from 2.0mm diameter enamelled copper wire (ecw), the dimensions of which are shown in Table 2.

All the dimensions were calculated using the formulae in table 1, which was, admittedly gleaned from “The Amateur Antenna Handbook” by William I Orr, W6SAI, although the beam is of my own design.

Table1

Polarisation
For horizontal polarisation feed from bottom for vertical polarisation rotate by 90◦

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To strengthen the elements of the quad a 2nd support can be fitted which will also make it easier to attach to the boom.

Making Waves – The Shorty Forty

September 17, 2017 Leave a comment

I was taking part in a Twitter conversation today with someone building the helical antenna published in this month’s RadCom, the RSGB member’s magazine, and having issues with the matching of it.  He was trying it out due to lack of space and a poor earth (clay) at his QTH.

I set to thinking and remembered an antenna design that I used to hand out to Foundation and Intermediate Licence trainees when I was mentoring them through their studies and assessing their practical assignments.  This was the Shorty Forty antenna because we don’t all have the requisite 20.28m of free space to string a dipole across.  I shared the plans with him and later thought “Why not share them for everyone?” so here goes:

The Shorty 40 – Helical Whip for 7MHz

So you want to get onto 40m but don’t have room for a dipole (20.28m)? Then this could be just the answer if you have a little time on your hands and enjoy home construction.

The Shorty 40 is a helical whip for 40m wound on a 3m long, 32mm diameter piece of PVC tubing (the sort available in most DIY stores). You will need 21m of 1.2mm diameter enamelled copper wire, 80cm of 2mm diameter ECW and 10 or 15m of 1.5mm diameter insulated copper wire (the sort used for lighting circuits or earth wire). You will also need a SO239 connector and a piece of angled aluminium.
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The picture says it all really but, just in case, begin by winding the 21m of copper wire along the length of the pipe, using tape or adhesive to secure it along the way. Cut the 80cm of 2mm ECW in half and push through the holes drilled in the top of the pipe. Solder together in the centre and solder the end of the coil here also. Drill a 16mm hole in the aluminium bracket for the SO239 socket and then attach it to the pipe using machine screws. Then solder the other end of the coil to the centre pin of the socket. Connect pipe to mast and connect two or three 5m radials to the solder lug on the aluminium bracket. Attach coax, raise mast and away you go.

Disclaimer  I cannot claim to be the first person to develop an antenna such as this but I have researched ideas on the internet and in books on the subject – from ARRL, PW Publishing and RSGB publishing – and changed them to suit modern metric measurements and make them easier to understand and build.