Users of the new half size G4CFY trapped dipole have had widely ranging results, quite inconsistant with any possible variation in production differences.   The factors affecting results seem to be related to resonance measurement equipment and aerial height.  The most succesful result required an addition of 4 inches to each of the inners of the top wire.  Then resonance was achieved within each of the bands as advertised, 40, 20, 15, & 10 metres.  Consequently future half size aerials will be supplied with an overall length of 54 feet.

Update 8 August 2009

100 Ohm high quality Twin Feeder.  The new high power rated twin feeder is meeting the approval of all who have purchased it. They are really impressed with its construction and performance. It is made using twin cores of 24/0.2 individually sheathed in polyethylene then with a solid moulded outer sheath of polyethylene.  The solid construction avoids water ingress and it has good flexibility.  The close spaced cores ensure virtually no differential noise pick-up.   It is typically 0.5dB per metre quieter than wide spaced feeder and coax.  On transmit the signal in the cores is equal amplitude but opposite in phase and cancels out any emitted field, thereby minimising TVI.  It must be used in conjunction with a 1:1 Balun.  Characteristic impedance 100 Ohms, 2kW power handling, 0.04dB/m loss at 10MHz (much the same as solid dielectric coax), weight 27gm/m.  £0.75/m plus P&P, or £70.00 for a 100m drum including P&P.  I charge just £5 to make-off both ends.

The smaller solid core twin feeder, actually 65 Ohms and rated at 150W is still available and weighs 23gm/m. 60p/metre plus carriage at cost or £55 for 100 metre drum including P&P.  I charge £7 to make-off both ends.

Update 17 September 2007

The Inverted trapped L aerial is half a trap dipole with the feedpoint at ground level.  It operates on 80 - 10 metres and is only 2dB down on a full quarterwave 80 metre inverted L.  With a total wire length of just 53 feet this can be accomodated in a very small garden, with say 20 feet vertical and just 33 feet back to a support on the gutter or chimney of the house.  This type of aerial is unbalanced and must be fed with coax. The complete aerial, top wire, two insulators, trap, and coupling box £82.00 including Special Delivery.

The design equations show that the coax traps having a higher L to C ratio than used in the W3DZZ design and need to be used with an inner wire length of 33 feet and an outer wire length of 19 feet 7 inches.  Some 2 feet shorter overall than as sold by G2DYM. Customers who have tried these lengths found that correct resonance occurred on both 80 and 40 metres. 

I have found an article from Technical correspondence in QST August 1976 which gives the equations to determine the net inductance loading required for a trap at a defined position on an aerial wire.  The extensive equations have been tried and do agree very closely with the W3DZZ design of trap dipole.  A program has been developed to calculate top wire lengths and trap inductance and capacitance.

Measurements of the inductance of the 7MHz coax traps reveal the inductance to be about 12uH.  The effect of connecting the inner from one end back to the outer at the other end effectively shorts out the inner to outer capacitance at low frequencies.  At 7MHz the isolating effect of the intervening inductance allows the capacitance to total around 42pF.

Introduction

On 30th October 2006 Spectrum Communications acquired G2DYM Aerials.  That business has run for exactly 30 years selling Trap Dipoles mainly to the Amateur Radio fraternity.  Richard Benham-Holman G2DYM the previous owner had just had his 84th birthday and due to failing eyesight so was unable to continue running the business. 

The G2DYM Trap Dipole.  This is based on the W3DZZ trap dipole design using a 7MHz dipole comprising two sections of 32 feet, with 7MHz traps then a further 21 feet 7 inches beyond the trap.  As standard it is supplied with 70 feet of 75 ohm twin feeder.  A optional balun is then used to convert to the unbalanced feed of the transceiver.

Full size Trap dipole

On 160 metres the two wires of the feeder are connected together and then in conjunction with an ATU the aerial operates as a Marconi T when tuned against Earth.

On 80 metres, the off-frequency trap acts as inductive loading and extends the effective length of the outer sections to 132 feet, the half wavelength resonance for 80 metres.

On 40 metres, the traps isolate the end sections so it works as a half wave centre fed dipole. 

On 20 metres, the off-frequency traps reduce the effective length of the whole aerial so it operates as a three half wave centre fed dipole.

On 15 metres again with the off-frequency traps it operates as a five half wave centre fed dipole.

On 10 metres it functions as a seven half wave centre fed dipole.

Full size Trap Dipole, 106 feet overall length, for 80, 40, 20, 15, & 10m parallel fed, also 160m as T configuration.  It comprising two inner sections of 33 feet and two outer sections of 19 feet 7 inches, two 7.1MHz traps, a centrepiece, two dog-bone end insulators, and 75 ohm twin feeder.  All the wires used in a complete dipole are terminated either by wrapping back round and soldering, or by crimped and soldered lugs.  All termination ends are properly finished with heatshrink and PVC sleeves.  Connections to traps and the centrepiece are made using M6 nuts and bolts.  The power rating is 400W continuous for all version.

Light duty, for sheltered environment.  2.5sq.mm stranded PVC covered top wire.   £124.00, not including feeder, see below for options.  £18.00.

Light duty, for sheltered environment.  4.1sq.mm hard drawn bare top wire.  £132.00, not including feeder, see below for options.  Carriage £18.00.

Medium duty, for typical inland site. 6sq.mm stranded PVC covered top wire.  £142.00, not including feeder, see below, for options.  Carriage £18.00.

Heavy duty, for exposed site, 10sq.mm stranded PVC covered top wire.  £164.00, not including feeder, see below for options.  Carriage £18.00.

 Half size Trap Dipole, 54 feet overall length, 40,20, 15, or 10m parallel fed, also 80m as T configuration. It comprises two inner sections of 16 feet 6 inches and two outer sections of 9 feet 3 inches, two 14.15MHz Coax wound and epoxy potted traps,  two end insulators, and a centre piece for low impedance twin feeder. All the wires used in a complete dipole are terminated either by wrapping back round and soldering, or by crimped and soldered lugs.  All termination ends are properly finished with heatshrink and PVC sleeves.  Connections to traps and the centrepiece are made using M6 nuts and bolts.  All versions rated at 400W continuous.

Light duty, for sheltered environment.  2.5sq.mm stranded PVC covered top wire.   £100.00, not including feeder, see below for options.  Special delivery £10.00.

Light duty, for sheltered environment.   4.1sq.mm hard drawn bare top wire.   £102.00, not including feeder, see below for options.  Special Delivery £10.00.

Medium duty, for typical inland site.  6sq.mm stranded PVC covered top wire.   £109.00, not including feeder, see below for options.  Special Delivery £10.00.

Heavy duty, for exposed site, 10sq.mm stranded PVC covered top wire.  £120.00, not including feeder, see below for options.  Carriage £18.00.

150W rated feeder, 75 ohm twin with ends made-off.  35ft £12.50.  70ft £18.00.  96ft £22.50. P&P £3.00.

2kW rated feeder, 100 ohm with ends made-off.  35ft £13.50.  70ft £21.50.  96ft £27.00. P&P £3.00.

Balun 1:1 ratio, 160 - 10 metres, air cored, 2KW rated. Low impedance in and out. £40.00, carriage £3.00.

3.5MHz G4CFY traps.  Under development.

7.1MHz G4CFY traps.  Coax wound, £30.00 each, post £3.00 singly, or £3.50 pair.

14.15MHz G4CFY traps. Cox wound, £30.00 each, post £3.00 singly, or £3.50 pair.