How to make a Weatherproof Vertically Polarised Omnidirectional Aerial

Introduction - The Slim Jim (Acknowledgements)

This is a vertically polarised  omnidirectional free space aerial (antenna) for 3 metres (FM broadcast band, 88 - 108MHz).  It can be scaled for Band I or UHF use.  Key features are

• Radiation efficiency 50% better than ground plane aerial, due to low angle radiation
• Unobtrusive
• No ground plane radials, so low wind resistance
• Fully weatherproof
• 50Ω input impedance
• Low VSWR - 1.5 to 1 or better
• Integrated balun

This design has been tested with 100W of transmit power.  No problems are anticipated for powers up to 500W.  The name Slim Jim comes from its slender construction and the use of a J type matching stub (J integrated match = JIM).  The integrated balun means that the balanced dipole of the aerial is properly connected to the unbalanced co-axial feeder cable from the transmitter.  If you don't use a balun, (for example if you connect your coax straight to a simple dipole), your coax turns into part of your aerial.  This is bad as

• Transmit power is wasted by being radiated from the cable instead of the aerial
• Increased risk of your RF (radio frequency) power interfering with your audio circuitry in and connected to your transmitter, due to RF being radiated from the feeder cable near to your transmitter.

Why is the Slim Jim so much more efficient than the popular 5/8λ or other ground plane aerials, despite the latter's claimed 3dB gain over a dipole?  The Slim Jim vertical angle of radiation is almost parallel to ground so maximum radiation is where it is needed, straight out and all around.  With all ground planes, including those with radials even one wavelength long, the vertical angle radiation is tilted upwards at an angle of 30° or more.

Construction Details

Summaries

• Construction Time (once you've done it a few times): 2 hours
• Test Time (once you've got the hang of it): 15 minutes
• Cost: Around £10.00
• Degree of difficulty: Moderate

Materials required

• 3m of 40mm diameter white* PVC waste pipe - from plumbers or builders merchant
• 3m of 19mm diameter plastic waste pipe
• 2 40mm diameter plastic waste pipe end caps - not unscrewable inspection cap type
• 6m thick (18swg, 1.25mm diameter minimum) single core copper wire, (tinned, enamelled or insulated)
• 25cm RG58C/U or similar 5.5mm diameter 50Ω coax cable
• UHF (SO239) or N-type 4 hole square chassis socket, (eg Maplin BW85G or FJ80B)
• 4 sets nuts, bolts, washers to suit the above socket (6BA or M3) and solder tags
• PVC insulating tape
• Insulating Sleeve

*White PVC has been demonstrated to have significantly lower RF losses than grey or black PVC.

Tools required

• Tape measure
• Hacksaw
• Heavy duty side cutters
• Sharp knife
• High Power (60W or greater) soldering iron and solder
• Screwdriver, nut runner (6BA or M3)
• Drill and drill bits
• Blow torch or hot air gun

Test Equipment required

• RF Power source (transmitter [Tx]) on your desired transmit frequency - preferably not too many watts. 1 to 5W ideal.
• VSWR Meter
• Two RF cables, short one from Tx to VSWR meter, long one from VSWR meter to aerial

The aerial is constructed from two pieces of plastic plumbing pipe.  The inner 19mm diameter pipe supports the wire that goes to make the dipole, This assembly is slid inside the outer 40mm diameter pipe which forms a weatherproof enclosure.

Scaling the Aerial to your Transmit Frequency

The reference length of the aerial for 98MHz is 2.205m (86.8 inches for Americans and other backwoods dwellers).  You will need to scale this figure for your transmit frequency.  If, for example, your transmit frequency is 88MHz, take 2.205, divide it by 88 and then multiply by the reference 98 to give 2.456m (96.68 inches).  If your transmit frequency is 108MHz, divide 2.205 by 108, and multiply by reference 98 to give 2.000m (78.77 inches).  This dimension is your overall-length.

You will also need to calculate the two other distances using the same technique.

1. Distance from the bottom of the aerial to the feed-point, reference dimension for 98MHz is 111mm (4.37 inches).  This is your feed-distance
2. Distance from the bottom of the aerial to the break in the wire, reference dimension for 98MHz is 710mm (28.0 inches).  This is your breakpoint-distance.

Having calculated your dimensions,

1. Cut the 19mm diameter pipe to your overall-length.
2. With the knife and sidecutters, cut 4 little slots into the ends of the 19mm pipe, just wide and deep enough to hold the thick wire in place.  Cut 2 at each end of the pipe, opposite each other, and in line with each other.
3. Starting between the lower two slots, measure up from the bottom of the 19mm pipe your feed-distance, mark the pipe here.  Cut a slot in the wall of the tube centred on this mark 20mm (0.8") long and 5mm (0.2") wide.
4. Measure up from one of the lower slots your breakpoint-distance, mark the pipe here.  This is your wire start-point.
5. From this mark, measure up 37mm (1.5 inches).  This is your wire stop-point.
6. Starting at the start-point, apply the wire to the outside of the 19mm tube.  Go down to the bottom, across the bottom, up the other side, over the top and come down the first side to the stop-point. Use the tape as you go to secure the wire in place.
7. If the wire you used was insulated, clear the insulation in the vicinity of the feed-point.
8. Now to prepare the waterproof sleeve - Make sure that the ends of the 40mm pipe are cut square.  Push one of the 40mm end caps into the end of the 40mm pipe.  You will probably need to heat the plastic of the 40mm pipe to soften it enough to get the end cap in.  Do this with the blow torch or hot air gun.
9. Take the other 40mm end cap.  In the centre of the flat face make a large hole for the centre of the RF socket, and 4 smaller holes for the fixing bolts.  Use the power tools and various other household and garden implements   Try to make a good job of it, as this is part of your weather seal.
10. Take about 250mm (10") of the RG58, strip and prepare both ends into pigtails.   Keep the socket end pigtails as short as possible.  The aerial end pigtails have to reach the two conductors on each side of the 19mm tube, so will be about 25mm (1")  long.
11. Solder the centre conductor to the inner of the socket.  Solder the screen to a solder tag.  Better still, divide the screen into two and solder to two solder tags.
12. Thread the RG58 through the hole in the 40mm cap.  Assemble the RF socket to the end-cap from the outside, using the nuts, bolts and washers.  The solder tag(s) have to go under the nuts inside the end-cap.  Much cursing and swearing usually occurs at this point. You should now have a piece of RG58 with 25mm pigtails at one end, and a 40mm end cap fitted with a RF socket dangling off the other end.
13. Thread the pigtails up through the slot in the 19mm pipe. Temporarily solder the inner to the uninterrupted side (no gap), the screen to the interrupted side.   All soldering operations must be conducted with a minimum of fuss, in order to avoid melting the insulation of the cable into oblivion.  Practice on another piece if necessary.

Tuning

You're now ready to start tuning.  To do this you will almost certainly need to be outside (unless you live in an aircraft hanger) and not too close to any building and trees.  Having said this you need to have access to mains power to run your transmitter and your soldering iron.  I used to manage it in my back garden, by clamping the aerial to the top of a step ladder.  The tuning should be done with the aerial fitted into its' weather-proof sleeve, so push it in.  Don't try and fit the lower end cap at this point.  Clamp the aerial upright as shown in the picture.   Connect the full length of your feeder, with appropriate connectors on each end, one end to your aerial, the other to your VSWR meter.  The VSWR meter is connected to your transmitter with another short piece of cable.

Remember when you're tuning the aerial you will be transmitting, so all usual precautions should be taken.  You don't need to transmit at full power, so if you can, turn down the power of your TX, or use a lower power one.  The VSWR of the aerial is not affected by the transmitter power.   You just need enough power get a sensible reading out of your VSWR meter.   This should be in the range of 1 to 5W.  It's also worth pointing out that you don't need to run any audio to tune the aerial, furthermore, you don't even have to have your stereo encoder running. All you require is an RF carrier.

To tune the aerial run some power into the aerial, and measure the VSWR.  Switch off, and adjust by sliding the feed points up or down together on the thick wires to find the point of minimum VSWR.  This corresponds to maximum transmitted power.  You will probably need to do this several times to find the best point.  A VSWR of better than 1.5 to 1 should be possible.  A lossy feed cable will make the VSWR of the aerial look better than it is.  Remember the tuning  must be done with the aerial upright, and the weatherproof cover fitted.  The optimum point should be within 20mm (0.8") of where you started. When you are happy you have found the best point, solder the feed wires on securely.  Finally push the lower end cap into the outer sleeve using heat to soften as before.  It is possible to get the cap off again using the same technique.

In Use

Remember, the more height the better.  Read up on siting.   If rain is expected, or for semi-permanent installations, wrap some self amalgamating tape (available from Maplin), round the junction between the feeder plug and the socket on the aerial, to keep moisture out.   Also wrap tape round junction of feeder cable on feeder plug to stop moisture getting into the cable.

Acknowledgements

This aerial is based on the "Slim Jim" aerial in the "Two-metre Antenna Handbook" by F C Judd G2BCX, published by Newnes Technical Books, ISBN 0 408 00402 9, now out of print.

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Last updated 25 August 2003