PA8W Amateur Radio

Wil, PA8W,  E-mail:           

Small doublet for 40m and 80m

First an important question: Why would we need a full size (almost 40m wire) dipole to be QRV on 80m?

In fact the main reason is that most Hams like the antenna to be resonant to get a nice match to our 50 ohm feeding system.
The size of a dipole however is of very little importance for the antenna performance, if we find a way to deal with the low radiation resistance and the reactance of a short, non-resonant antenna.

Look at the following "ideal" 80m dipole: 

The halve wave dipole @20m height on 3.6MHz

The halve wave dipole needs approx. 39m wire, and in this model we put it up at 20m, which is very hard to realize or even impossible for most Hams.

The take off angle is perfect for traffic up to about 2000km.
For real DX, you want a lower take off angle, with a big dip at 90 degrees for good attenuation of the more local signals. Only a very big horizontal loop (>160m), or a vertical (with lots of 40m radials...) can provide this. 
Not within the possibilities of the Ham with limited space.
The quarter wave doublet @7m height on 3.6MHz:

This quarter wave doublet is the antenna I used for 6 months for 40 and 80m.
The simulation shows only little performance degradation compared to the full halve wave dipole!
The only drawback is: you need a symmetric feeder and a good transmatch to tune out the reactance.
My transmatch is homebrew, and will be discussed on another page.
The feedline was about 5 meters of symmetric 300 ohm cable.
Tuning was rather sharp, but really not a problem.

Listen to my best report with this antenna on 80m:
Its seems that the low position of the antenna gives some extra gain , but this is a flaw of the modelling software MMANA, which calculates increasing gain at heights lower than 0,2 wavelengths, where gain should begin to decrease... The true gain will be around 4 dBi, so that's less than 1,5dB below the full size dipole @ 20m height.
But take note of the extreme mismatch: Z=2.012 ohm -j1219!
This will require a good feedsystem!
However, the pattern shows plenty of high angle radiation, making this antenne a typical Near Vertical Incidence Skywave antenna, and even with a few dB's of losses, this antenna remains very suitable for distances up to about 1500km. 

Here's my contacts on 80m with this quarter wave doublet antenna, in the first two weeks (oct.2009)
Note the contact with the westcoast of Greenland!
The quarter wave doublet @ 7m height,  on 7.15MHz:

The same quarter wave doublet works quite nicely on 40m as well.
Since it is almost a halve wave dipole, the tuner has an easy job, and tuning is rather uncritical.
This antenna gets me REALLY nice signal reports up to 5000 km distance. The front to side ratio becomes more distinct, but stays within one S-point, so it doesn't pose a dramatic problem.

And here's my contacts on 40m in the summer of 2009.
Note that several lines end at the other side of the globe...


Conclusions so far:

As the above experiments and simulations show, the performance of a dipole does not depend greatly on size.
The only parameters that change rapidly when leaving the resonant length is the radiation resistance and reactance of the antenna.

The actual performance and radiation pattern show minor changes only, so you may easily half the size of a dipole and get less than 0,5dB degradation.  
The drawback of the short doublet is the dramatic mismatch to 50 ohm. You need a good symmetric feedline and a good symmetric link-coupled transmatch to keep losses to a minimum, because if performance is degraded, it will be in the feeding and matching system. In this particular case the feedline and tuner losses may be somewhere around 6dB on 80m!

As a bonus this antenna design is easily tunable on 40m and 20m, with reasonably consistant radiation patterns.

And last but not least the balanced design improved my signal/noise ratio on reception quite a bit:
With an off centre fed dipole running over my shack, my laptop's switched power supply raised the noise level on 40m between 6 and 12dB.
Now, with the balanced doublet hanging at the same spot, I barely notice any noise of this power supply anymore. Super! 

But, as a radio amateur, I always wonder if things can be improved any further... 

Improving performance:
(=reducing losses...)

Ok, let's tackle the inavoidable losses:

1, We reduce ohmic losses by increasing the wire size from 0.75mm2 to 2,5mm2
2, We raise the antennas feedpoint impedance by adding 5 extra meters of wire per side, which may run around any available non conducting object.
3, We raise the antenna height by lifting its feedpoint from 6 to 10 meters above ground, thus increasing feedpoint impedance a little more, and since the antenna will become more like an inverted V, the 40m radiation pattern will be more circular, filling in the gaps in my favourable directions (to middle east and north America)
4, We pimp the feedline; 2,5mm2 wire, 5cm spacing, instead of 300 ohm ribbon.

On the right the new (current) situation:

The roof is the top of my shack, 
at a top height of about 5,5 meter.

The feedpoint is at approximately 10 meters height. 

The Spiderbeam telescopic fiberglass pole I use for this antenna is 10m high.
Starting off on the 2m high carport roof, the top height is 12m. 

The feedline is in fact an extention of the antenna wire, 2,5mm2 so plenty of copper inside.
The 50mm spacers are plastic tubes of a heavy type of summer curtains, which are used in the back entrance opening of the house to keep flying bugs out.
The components of these curtains can be ordered separately, in lots of colours, so I ordered 50 tubes in black.
Costs: 2,75 euro...

I use a black cable tie to fix the spacer:
First I stick the cable tie through the tube, around one wire, back into the tube, and on the opposite side I close it around the second wire.
Pull tight, cut off the excess of the tie and you're done!

I took a few days of comparing the two antennas before I was ready to remove the shorter, lower one.
Be aware not to tune both antennas at the same time because they will couple heavily, distorting the performance of the other one.
So, unhook the antenna not in use. As a non-resonant set of wires, it will seem non-existant.

Results on 80m:
Reports vary between no difference and + 10dB in favour of the bigger antenna.
This is consistant with my findings during reception, however, I never saw equal performance, but aways a better signal on the bigger one, on average around 6dB.
A surprise was that the bigger antenna also shows less local noise. 
This may be because of the increased distance to house wiring.

Results on 40m:
On 40 I also noticed a good improvement; 3 to 6dB improvement in my signal, also on reception, the higher values for those areas NOT broadside to the antenna. Nice!

Results on 20m 15m and 10m:
For the higher bands this antenna also does a reasonable job, but my loop antenna on these bands always wins hands down.
Being a little higher up in the sky, and rotatable, the loop is the best thing if you don't have any room for a beam.

The improvement of performance of this new doublet clearly shows that a lot of efficiency can be gained by:
1, adding some length, so impedance and reactance are less brutal to the feeding system, resulting in much lower losses.
2, using increased wire size for antenna and feeder to further decrease ohmic loss.
3, raising the feedpoint hight so that ground losses decrease a little more.

Having done this I found that my signal is at least comparable to the guys with full size dipoles.
In fact, in any local 80m net I participated, my signal was amongst the better stations, beating many guys with 6dB more power.
(Of course, good ssb audio is a highly underestimated factor as well...)

73, Wil.