The G3NGD Semi-vertical Trap Antenna
for 1·8 MHz, 3·5MHz and 7·0MHz
Mk.V1 - December 2003
 
Vertical section showing Mast coupler & guys.
The 7·1 MHz trap coil can be seen in the horizontal section running to the left.
Pot Insulators are fitted in each guy rope to improve the insulation at radio frequencies.
A pulley and halyard can be seen attached to the top of the vertical, which runs down to the left.
This is used to hoist up one end of a Triband Trapped Dipole Antenna for 20/15/10 Metres which
normally runs between the top of the Vertical and the gutter of the house. John only uses these
bands on the odd occasion, and so it is only erected when required.
For details of John's latest idea (Inverted 'V' Antenna) using this pulley and halyard click this link
 
 The original Antenna was designed by G3NGD in 1975.
Many changes and improvements have been made over the years, and this
Website shows the latest developments to date.
Since his Short-wave listening days (SWL A1456), John has always shown
a keen interest on the 160 Metre band. ("Top Band" 1·810 - 2·000MHz)
In addition to constructing many "Top Band" transmitters and receivers, he
always tried  to compete with other Radio Amateurs in  putting out a strong
signal although his garden had always been too small to accommodate a good
aerial. The ideal aerial for transmitting is a half wave dipole, and for this band
the length would be approximately 264 ft long.
John's main desire was to communicate "across the pond" - across to Canada
and  the USA using this band. The band is normally used for  local contacts during the daytime, and communication for the whole of the U.K. at night.
During the day, there  is only a ground wave available (50 Miles radius), and so long distance is only possible using Sky wave during the hours of darkness.
At that time (1974), the prospects  looked grim, but he  who persists usually
achieves success.
 
 QSL card to confirm contact with Canada on "Top Band" using Morse Code.
QSL card to confirm contact with USA on "Top Band" using Single Sideband.
 
Another QSL card from Leverett, Western Massachusetts, USA, using Single Sideband
 on "Top Band" 1840 KHz.
  The antenna is still used by John, and over the past twenty five years 
  has radiated signals into many countries of the world.
     
    Just to show that the signal goes to the East as well as the West!
    The performance on  3·5 MHz (80 Metres) and & 7·0 MHz (40 Metres)
    is equally as good:
    3·5 MHz - CN8,  W2,  JX2,  WA8,  YV5,  JA6,  VO1, VP1,  W9, OX, HK4,
                      VU2, VP2,  6Y5, all on  SSB.
    7·0 MHz  - EP2,  Y22,  OJ0,  UK9,  N2,  K3,  W1,  W4,  VK3, all on  SSB.
      (28/10/00 contacted W4MYA, W1FJ, K3LR, KQ2M on 7MHz (SSB) - all R5 - S9)
      May 2001 - TA3, SV9, RW6, 9M6, T95, CX5, PY6, 4N8, OY9, I2, EA4, CE8,
                                  D44, 9G0, LU2, ZP5, YC3, CN8 all on Single sideband (SSB).
    For the novice:        LU2 is the Callsign Prefix for Argentina
                                     ZP5 - Paraguay,          CE8 - Tierra del Fuego (Chile)
                                     D44 - Cape Verde,     CX5 - Uruguay,
                                     VK3 - Australia,          W    - USA. 
                                     9M6 - Sabah (Malaysia) 
    You can find the location of Radio Amateurs  by clicking your mouse on the website: http://www.qrz.com
 
Construction details
 
The aerial arrangement
The vertical section is constructed from two 12ft lengths of 2in dia aluminium tube, joined together with mast couplers. The dimensions are as shown above.
On the latest Version (Mk.IV),  Pot Insulators are fitted on each guy rope. 
After a period of time, soot (carbon deposit) forms on the 'Poly chord' which could introduce R.F.signal losses.
 
The base insulator, mountings, and feed point connection
  Please read on to find details about the modifications.
The feed insulator is made from 2in dia vinyl drainpipe which was found to be a
push-fit  over the aluminium tube. This insulation is sufficient as  the aerial is a
quarter wavelength long on  all three bands, and the impedance at  this point is
very small. The current is therefore a maximum and the voltage should be zero.
 
The 7·1MHz trap assembly
The 7·0MHz trap is wound with 20 turns of 20 swg copper wire spaced by one turn to prevent arcing, on a 6in length of 1½in diameter vinyl tube. 
The coil has a 50pF capacitor rated at 3KV wkg wired in parallel across it.
It is vital that the tuned circuit is resonated at 7·1MHz using a dip oscillator. As a dip oscillator calibration is usually poor, it is best to measure the dip oscillator frequency at dip (resonance) using either a digital frequency meter or the station receiver.
The coil is protected with a 6in length of 2in diameter vinyl tube, placed over
the  coil assembly.  The ends are sealed with  fibreglass filler, as shown above,
the fibreglass ends being held in place with four brass screws.
 
The 3·7MHz trap assembly
The  3·7MHz  trap assembly is  similar  to  the 7·1MHz  assembly as  shown above.
The coil is wound with 40 turns of 20swg copper wire spaced by one turn to prevent arcing. The mica capacitor is 75pF at 3KV wkg. 
Note: two 100pF pulse capacitors at 2KV wkg (used in a Television receivers line output stage) could be used when connected in series. 
The resonant frequency of the trap should be adjusted to 3·7MHz with the aid of a dip oscillator.  This  is  done by winding more turns on the coil than stated, then subtracting the number of turns until the resonant frequency is achieved.
Note: John resonated the trap on 3·7MHz because he wanted to work mainly D.X.
(Long distance), Single Sideband, which usually appears at the H.F. end of the band.
If an operator uses C.W. (Morse Code), then it would be better to resonate the trap
in the centre of the 80 Meter Band.
The wire used for the top sections is 2·5mm² pvc insulated copper wire.
Length (a-b) is 10ft in length and length (c-d) is 26ft in length.
Length (e-f) is approximately 24ft in length, and depends on the highest resonant frequency required on the 160 Metre Band.
The Mk.IV version resonates at 2·0MHz when on 'Top Band', and the lower
frequencies are tuned using a "Roller Coaster" at the feedpoint of the vertical section. (Between the 'Matching Unit' and the base of the vertical).
 
This Roller Coaster is fitted on top
of  the Impedance Matching Unit,
in series with the lead attached to
the feed point of the Vertical tube.
It is connected when on 160metres
General Erection.
A site should be chosen approximately 10 ft from the bottom of the garden, and a 5ft
length of 2in diameter aluminium tube fixed vertically in the ground with concrete to a
depth of 4ft. This leaves 1ft protruding above the ground which when sleeved with 2in
diameter vinyl pipe forms the base insulators.
Guy  clamps  should be fitted on  the vertical, and  polypropylene  rope attached with
anchor rings. This rope is sold at "Wilkinsons Stores" as  clothesline,  (£1.49 for a
30 metre length), or try B&Q DIY.
On the latest Version (Mk.IV), Pot Insulators are fitted on each guy rope. 
After a period of time, soot (carbon deposit) forms on the 'Poly chord' which could introduce R.F.signal losses.  - [This was done after reading an article in Radcom.]
Earthing.
There are two earth rods, each 5ft long which are hammered into the ground and wired
with copper wire to a point underneath the base insulator. It is an advantage to connect
as many radial wires as possible to this point, but not absolutely necessary.
Coaxial Cable.
The coaxial cable used is Uniradio 43 (50W)  and is connected braid to earth and inner
to the feed point (50W - see below) on the toroidal matching unit. This unit is built in a
Diecast Aluminium box and mounted on the ground at the base of the vertical.
The cable is fed from the shack to the matching unit  inside a length of  20mm diameter plastic conduit or a length of blue water pipe buried 2ft deep.
It would be even better if copper tube was used as this would provide an excellent earth.
 
Below is the circuit of the toroidal auto-matching transformer using an Amidon T157-2 ferrite ring core available from "Cirkit Electronics". This is fixed at the
feed-point at the base of the vertical section.
The switch wiper contact feeds the  base of  the  antenna  and
provides an impedance match
in the range 12·5W to 50W.
When on Top Band (160 metres)
the wiper is connected to the
lower end of the 'Roller Coaster',
whilst the 'slider' of the 'Roller
Coaster' goes to the base of the antenna. 
The transformer is wired with 2 x 20 turns "bifilar wound"  18swg enamelled
copper wire. Both windings are connected in series, in phase. The secondary
winding is tapped every other turn and a wire from each tap is connected to
a contact on a "RS Components" 1-pole, 11 way wafer switch. The assembly
is as shown below. It is built in a Diecast Aluminium box and mounted on the ground at the base of the vertical. 
When using 3·8MHz and 7·0MHz, the switch wiper can go directly to the base
of the vertical. When using 1·8MHz, the 'Roller Coaster' is wired in series
with the switch wiper and the base of the vertical. (It should be noted that, if
the 'Roller Coaster' is tuned to its first turn, then it could be left in circuit).
 
Setting up.
When the aerial is erected, the only adjustment necessary should be for 1·81 -  2·0MHz.
With the transmitter tuned to 2·0MHz  (or the highest resonant frequency required), the length e - f  should be tuned to give an swr of 1 : 1 using an swr bridge.
Checks should then be made with distant stations to establish whether or not the signal decreases in strength on either side of resonance. Start with the Matching Unit switched
to the 50W (position 1) with the 'Roller Coaster' slider on the first Turn of the coil.
Adjust the switch on the Toroidal Matching Unit until the swr is the lowest value or (1:1).
When using other frequencies between 1·81 -  2·0MHz rotate the 'Roller Coaster' for  the best
match. It was found that: 1·830MHz resonated swr 1:1 with 16 turns on the 'Roller Coaster';
1·900MHz resonated swr 1:1 with 11 turns; 1·950MHz resonated swr 1:1 with 8 turns. 
On 3·5MHz the swr is 1 : 1 on the resonant frequency of the trap and cannot be altered
by adjusting the length c - d. A useful bandwidth of about 100KHz is possible.
The bandwidth on 7·0MHz is wide due to the large diameter of the vertical radiator.
The best impedance match using the Toroidal Matching Unit,  for each band is as follows:
'Top Band'                 1·810 - 2·000MHz   -   28W  (Position 6)
'Eighty Metre Band'    3.500 - 3·800MHz   -   21W  (Position 8)
'Forty Metre Band'     7·000 - 7·100MHz   -   15W  (Position 10) 

 
     General notes of interest noted after experimenting.
It should be remembered that on a quarter-wave aerial, the signal
radiates best between the feed-point and the lower  section of  the vertical, where the current is a maximum and  the Impedance is a minimum.  It doesn't radiate from the far end of  the aerial where 
the Voltage and the Impedance is a maximum.

If a coil is placed at the feed-point, to give a good SWR, when the
aerial is used at the low frequency end of the 160 metre band (in 
order to make the aerial resonant), the current maximum point
is moved. This negates the purpose of using a vertical aerial some-
what. If one only wishes to use the LF end of the 160 Metre band,
then it is better not to use the roller coaster, but  to lengthen the 
section e - f.   If one hasn't got the room to extend this section,
then one can wind a coil on a PVC tube, to extend the length. The 
coil should be placed immediately after the 3·8MHz trap. (This has
been tried and works well.) The impedance at the base was found 
to be approximately 28W (Position 6 on the base matching unit).
When using the roller coaster to resonate, the base impedance was
found to be 25W (Position 7).

It was suggested by another Radio Amateur to resonate the Aerial
on 1·850MHz to provide full vertical radiation for DX, and then to
use a capacitor to shorten the antenna for working at 1·950MHz.
This was tried using a 500pF air-spaced variable capacitor and 
worked successfully up to approximately 1·920MHz. It was found 
however, that as the frequency was increased above 1·920MHz.,
the feed impedance also increased to over 50W. The matching unit
would not bring down the SWR as its impedance range is limited to 
between 50W and 12·5W.

As John uses both ends of top band, the capacitor idea was rejected.
The feed-point impedance remains as: 28W (Position 6) for the HF end,
 and 25W  (Position 7) for the LF end (using the 'roller coaster').

 
A reminder!
This aerial was originally designed so as to work effectively in a relatively small garden (50ft).   In the original design, the 7MHz Trap coil was fitted 
on top of the vertical section which was 33ft high. This webpage only shows the modifications. The original design can be found in my Article written in Radcom, August 1976 or in CQ Magazine January 1977.

 
Just to confirm that the antenna works at least as well as the original version (1976) which was 33ft vertical,
with the 7·1MHz Trap coil mounted on the top - John has worked 38 countries on top band during January,
February and March this year (2004), all on SSB. These countries included Morocco, Greece, Azores, Italy,
Cape Verde, and Canada.

 
A final note of interest.
Although the antenna was designed for use on Top Band, Eighty and Forty Metres only, it
has been successfully used on 14MHz (20m), and 21MHz (15m) without modification.

When compared with John's 'Triband Trapped Dipole Antenna', suspended between the top 
of the vertical and his house, it doesn't perform as well. It does however, radiate signals 
into both the USA and Australia at a strength making a 2-way SSB (100W) contact possible. 
It is useful to be able to make general contacts, without having to erect the main HF antenna.
G3NGD uses the low frequency bands most of the time as he has worked the world over the 
past 45 years of operating.  He finds it nice to work the world now on the LF bands.

When the Vertical Antenna is used to Transmit and Receive on the 14MHz and 21MHz, bands
the 'Roller Coaster Inductor' is switched out. The full length of aerial is used, and the appropriate impedance is selected on the Toroidal Matching Unit. The impedances being 
40W (Position 3)and 50W (Position 1) respectively.

An SWR reading of 1:1 can be obtained without using an 'Aerial Tuning Unit' at the transmitter.

Please note: John decided in May 2004 to try his 'Triband Trapped Dipole Antenna' as an
inverted 'V' by suspending it from the top of the vertical radiator. This is more convenient
than using the original idea and is in current use at this time. It is working well!
(c) G3NGD 2006