TheRadioBoard

Due to limited space the maximum length of my antenna can only be 5meters (16.5 feet) on the attic of my house. Last night I did some experiments to see if I could improve my antenna. As you all know with crystal radios every pW counts.

At 1MHz a 5 meter antenna is only 1/60 of the wavelength and so short that it behaves like a capacitor so it only receives the E-field. Increasing the area of the antenna should increase capacity and catch more E-field yielding more power from the antenna. To increase the antenna area I added a 5meter long and 30cm wide tinfoil sheet to the antenna wire. As expected the capacity increased and I had to retune the antenna tuner/matching unit. To my surprise the signal strength didn't increase .

Next I tried a second wire parallel to the first (tinfoil removed) at a distance of 90cm (3 feet). This again increased the antenna capacity but now I had about a 30% increase in signal strength . Next I added two more wires with each wire separated by 90cm. Again more antenna capacity and over 100% increase in signal strength over a single wire . Apparently each wire gives about 30% increase

In both tinfoil and wire experiments the antenna capacity increased but only by adding more wires I was able to increase the signal strength.
I don't understand why this is the case any suggestions?

Corne

eddy currents in the foil is my guess.

Remember that antennas are black magic! But what you were doing is possibly lowering the total impedance of the antenna at the feed point which was showing up a better match to your receiver. You were also increasing the "capture area" of the antenna, and that would also explain better signals being received.
Curt

_________________
Curt, N7AH
CW forever
Connoisseur of the cold 807

I'll make a stab at this. This is only my conjecture, not an attempted explanation to the 'black magic'.

I don't think the 30 cm wide tinfoil really adds anything to capture area in that small of a length at BCB. Yes, it would add capacitance but capacitance=signal strength is not equated.

The additional wires at 90 cm apart DO add some capture area in comparison. They are distinct and they add signal differently than one single solid conductor (like the foil) would. They are only a tiny bit out of phase so they should add.

Look at the old 1920s MW antennas. Cages and multiple conductors spaced a few feet apart. Modern explanation often tends to dismiss that as simply a large top hat that provides capacitance for the vertical 'lead in'. If V/H polarity comes into question it should be noted that most early BC stations used a similar horizontal configuration.


-Bill

If the collecting area has influence on signal strength I should get more signal from two wires at 180cm apart than I would from 90cm. So I tried two wires 180cm apart. I had to retune the antenna tuner about the same as I would for two wires at 90cm. So the antenna capacity for two wires at 90cm and 180cm are about the same. The increase in signal strength was 30% so exactly the same as with two wires 90cm apart. Adding the wire at 90cm, so now I have 3 wires spaced at 90cm, increased the signal strength by about 70% compared to a single wire.
In all the experiments I did I got about 30% increase for each additional wire if the spacing is 90cm.


Sure seems that way.

I'm happy that I found a way to significantly increase signal strength without increasing the antenna length.
But I'll have to look into the matching between the antenna and receiver to see if I can optimize it.

Corne

Corne,

I run a "clothesline" antenna. 4 wires at 60 feet long spaced about 3 feet apart. They work good if you have a short space and there is no way to go longer.

There must be a diminishing return at some point. The last wire I put on did not give that much more of a result so I stopped adding. It would be interesting to see where it actually stops.

DB

Darryl,

I see you are using a similar spacing than I do (3feet=90cm) only your antenna is 3.7 times long than mine.



With me the 4th wire added about the same as the 3rd.



I'm not going to find out where it stops if it does at all because I only have space for 6 or 7 wires before my attic space runs out.

Corne

Corne,
I have enough problems with spider webs in the attic!

If I had 6 wires in my attic, I'd have to have the fire department come in and cut me loose after I got tangled up in it!

Mine is on the roof. and looks like a clothes line from the back yard.

DB

This morning just after I replied to an email about the subject it hit me. I shouldn't look at the voltages but the power generated by the voltages.
Yesterday I redid the multi wire antenna measurements trying to understand what was going on. With one antenna wire I got 73mV. Adding the second wire increased the signal to 103mV, the third wire got it up to 124mV and the fourth wire gave 149mV.

The load during the measurements was 200k so
0.073^2/200k= 27nW
0.103^2/200k=53nW
0.124^2/200k=77nW
0.149^2/200k=111nW

Each wire adds about 26nW. Finally this makes sense.


Corne