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Since 22.05.2006
Depuis le 22.05.2006
Dal 22.05.2006

Vertical Vacation Antenna for 80, 40 and 20 Meter-Band, by Herbert OE3BMB
This antenna is a full size λ/4 for 20m and 40m band, and a shortened λ/4 for the 80m band.

Generally on the 40m band the SWR is under 1,6. In 80m and 20m band at the resonant frequency the SWR is about 1,1.  I aligned for 3,700 kHz and for the CW-range I insert a 10 cm piece wire between feed and coil.

What do you need?

1 piece aluminium metal sheet, approx. 8 x 8cm, minimal 1 mm thickness
1 piece of plastic pipe 50 - 55 mm diameter and approx. 20 cm long
(both hopefully for free at the plumbers)

1 PL-Socket
1 hose clamp for approx. 7 cm pipe diameter
1 earth peg (in the building market available)
1 fiber glass telescope mast

50 cm stranded wire 2,5 mm² for bypass
approx. 2 m stranded wire 2,5 mm² for spool,
approx. 9 m stranded wire 2,5 mm² for antenna
approx. 55 m stranded wire 1,5 mm² for radials (2x18 + 2x9m)
approx. 3 cm copper wire 2,5 mm² (no stranded wire)
approx. 50 cm double sided stick tape, approx. 5 cm broad

at least 10 pieces of round plugs, male and female (for wire Ø up to 2.5 mm²)...
..... and much insulating tape

Example for the "round plugs"

 

Construction instructions - PL-Socket for feeder

Ground Plan for PL-Socket

Mark first the aluminium sheet with a line in the centre. Saw in here the sheet from both sides approx. 2 cm. Drill 20 mm of the edge in the centre a 15 mm Ø hole as it is shown on the sketch. Make four small holes near the edge at the hole side*), where the four male round-connector sockets have to be soldered (remove insulation). The heads of the cases are to show outwards. See illustration. Now fix the PL-socked (the antenna connector must be on the same side as the radial-connectors), by drilling the holes and fasten it with blind rivets or small screws. (Proceeding in reverse, the soldering of the insulation of the PL-socket becomes too hot). Solder to an approx. 3 cm copper wire (no stranded wire) a male round plug. SolderThis to the PL socket center. Now bend the aluminium piece at the cuts.(see picture).

PL-Socket for Feeder

*)The diameter of the holes should have just the size, that the round plugs (without insulation) could e inserted.

 

 

 

 

 

 

 

Construction instructions - Coil

Now the coil must be winded. Stick on the plastic tube double-sided tape along 2 strips (better 3). Wind there - completely closely - 2.5 mm ² strand wire to 33 turns (at a coil of 53mm Ø) or 36 turns (at a coil of 50 mm Ø). I made 2 turns more, which I unreeled (if necessary) at the alignment. At both sides should be left approx. 20 cm. Strip the wire at the ends. Afterwards prepare a 9 m (antenna) and a 50 cm (bypass) piece of 2.5 mm ² strand wire.( strip the wires at both ends.

 

 

 

 

Assembling

Earth Peg with PL-Socket

Fix the piece with the PL-socked by means of a hose clamp to the earth peg. The thread must show downwards. Drive out the telescope mast and put it down. Fasten the coil approx. 25 to 30 cm from the l end with a tape to the mast. Then connect the antenna wire with the bypass line provisionally by means of block clamps, alligator clips, or something similar. Lead he antenna wire to the top, and fix it with tape severaltimes to the mast. Screw in the pivot peg, insert the antenna mast. Attach the antenna to the bypass wire and attach it provisionally to the PL-connector. Connect also two radial with approx. 18 m and one with approx. 9 m and spread them out on the soil.

 

Tuning

An additional SWR Analyzer is of advantage, but it will work with a measuring bridge (SWR meter) as well. In order to avoid troubles, align with lowest power.

First determine the point of resonance on 40 m band. If it is to low, shorten the antenna about 10 cm. If it is too high, length it with 10 cm at the top. I achieved a SWR on the entire 40 m band of 1,5. (the impedance was 35 Ω). (You can replace one long radial with a 9 m radial, but only for the 40 m band)
For 20 m band just cut the antenna wire after 4,1 m (including bypass wire it’s 4,6 m) at the feeding side.
Now remove the bypass and insert the coil.. The resonant frequency is probably below the desired one, because we have some turns too much. In this case remove a turn and shorten the wire. Measure again. Remember that shortening the wire on the feeding side naturally raises more the resonant frequency, as if it shortened on the antenna side. Repeat this procedure until you get the lowest SWR on the desired frequency. Never shorten the antenna wire or the bypass wire (otherwise the 40m-Band is detuned). (I adjusted to 3,700 kHz and use for the CW range a piece of wire between fitting bush and coil). If the SWR is not below 1,2, put away the shorter radial. One of the longer radials can also be extended. (Simple try).
Now replace the Provisional connectors with male and female round plugs.

Attach the transceiver, ground it, and go!

Concluding remarks

With inserted pipe

To work more comfortable and for a better radiation I inserted an approx. 1.5 m long pipe between earth peg and mast (see photo). The pipe serves as a transport case  for the telescope mast. Being able to change impedance and thus the SWR you may raise one or more radials on the ends. A tuner is recommended. Result: On 80 m the antenna obtains usually better results as a mostly too low positioned dipole in the vacation or fielday QTH. On 40 m and 20 m it is equal the usual groundplane antennas. There is still room for improvements, and I am grateful for relevant suggestions.

all components together

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

My E-Mail Adress:
herbert.mitsch@gmx.net

Herbert Mitsch, OE3BMB
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