Continued work on traps has revealed that using coax, with the outer one end connected back to the inner the other end, makes them polarised, or assymetrical.   The 7MHz traps change frequency by 20KHz driving one end instead of the other.   This is small enough not to be a problem. The traps for the higher bands are affected to a much greater degree, as much as 200KHz with the 14MHz versions.   Consequently all traps for frequencies above 7MHz will be made using discrete coil and capacitor.  Some my use the Reyco style, which has concentric tubes to form the capacitor with a moulded centre section on which is wound enamelled or PVC covered wire.   Alternatively I might use a 34mm (1.3") diameter plastic tube wound with enamel or PVC covered wire and a custom made low temperature-coefficient capacitor in the centre.

Due to the switch from coax wound to discrete wound 14MHz traps, the half size G4CFY dipole has increased from 54 to 56feet long.  also its price for all version is reduced by £10.

In addition to the Inverted L introduced in September 2007, I have now worked out prices for plain dipoles, made to the same high engineering standards as the trap dipoles.  Prices not including feeder or P&P at £7.50 are as follows;

75 Ohm Twin Feeder 15p/foot plus £19.00 to make-off with lugs and heatshrink.

Update 28 January 2008

Sales of G4CFY trapped dipoles and Inverted L antennas is steady and the reports back from the customers very pleasing. Traps resonant on 14MHz have now been produced and half size antennas for 40 and 20metres are selling slowly.  Most customers still want to operate on 80 metres, even with a small plot, and it seems that the Inverted L is the best solution. A new form of custom made capacitor with a 5kV rating has been developed. This allows traps to be made using a very wide ratio of L to C so that the length of a dipole can be tailored to fit the available space.

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.

Update 17 July 2007

A 95 foot, 4 trap dipole for 80 - 10 metres was produced in June.  I have now run the calculations with a higher L : C ratio in the traps and have a design now just 81 feet 5 inches long.  Using medium diameter top wire the cost is £261.00 including 70 feet of feeder and including Interlink carriage. 

Update 19 June 2007

Manufacture of custom and standard aerials is steady and quite encouraging.  The old molds are being scrapped and new ones brought into use to improve the surface finish of the traps.  The use of the complex design equation in an Excel spreadsheet together with a costing spreadsheet now enables realistic pricing of custom aerials to be quickly achieved.

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 total 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.

75 Ohm Twin Feeder

75 ohm twin feeder is lower loss than coax.  It allows the aerial to be properly balanced and the very close spacing of the wires prevents pickup or radiation from the feeder.   It does not need to be spaced off, unlike ribbon feeder.  Use of twin feeder makes this aerial much lower noise than one fed with coax.  Also importantly it generates less TVI  !!

Note that the aerial is generally 72 ohms, and will need to be used with an ATU with transistorised rigs which are unforgiving about SWR mismatches.

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.  Interlink Carriage £15.00.

Light duty, for sheltered environment.  4.1sq.mm hard drawn bare top wire.  £132.00, not including feeder, see below for options.  Interlink Carriage £15.00.

Medium duty, for typical inland site. 6sq.mm stranded PVC covered top wire.  £142.00, not including feeder, see below, for options.  Interlink Carriage £15.00.

Heavy duty, for exposed site, 10sq.mm stranded PVC covered top wire.  £164.00, not including feeder, see below for options.  Interlink Carriage £15.00.

 Half size Trap Dipole, 56 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 9 inches, two 14.15MHz Reyco style traps,  two dog-bone end insulators, and a centre piece for 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.  All versions rated at 300W 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.  Interlink Carriage £15.00.

Feeder, 75 ohm twin with ends made-off.  35ft £23.50.  70ft £28.00.  96ft £31.50. 108ft £33.00  128ft £35.50.  140ft £37.00.  P&P £3.00, (unless included with top wire).

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.  Wire wound with high voltage capacitor, £40.00 each, post £3.00 singly, or £3.50 pair.

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

14.15MHz Rayco style traps. Wire wound with concentric tube capacitor, £25.00 each, post £3.00 singly, or £3.50 pair.

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