While contemplating the design for an indoor AM Part15.219 compliant antenna I found it necessary to measure the RF impedance of a 3 meter length of wire in order to spec. the loading coil. A 30 gauge wire was used and was strung horizontally in my basement lab as far away from objects as practical.
The test setup consisted of an RF signal generator, scope, and a RF current transformer. The generator was connected to the input of the transformer and the voltage was measured at this point with the scope. The current signal from the transformer was connected to the second scope channel so the amplitude and phase of both the current and voltage could be observed. The ground of the current transformer was connected to the power line ground and the antenna was connected to the output of the transformer.
A series inductor of 150 uH was connected in series with the antenna and the frequency was adjusted until the system resonated as shown by zero degree phase shift between V and I. The V and I were measured and from the frequency and known inductance the antenna impedance was calculated.
With the 30 gauge wire the Z = 108 -j2626 ohms which corresponds to an antenna capacitance of 22 pF.
This capacitance seemed rather low so the wire was replaced with a copper pipe of .8 meter length of 3/4" telescoped around a 2.2 meter length of 1/2" pipe giving 3 meters overall length. The pipe was positioned exactly where the original wire was located.
The impedance with the pipe was Z = 126 - j1933 ohms corresponding to antenna capacitance of 30 pF.
This experiment indicates that there may be an advantage to using a radiating element larger in diameter than a wire since it increases the capacitance and thus lowers the inductance needed for the loading coil. The loading coil is a major source of power loss in such an antenna system and the less wire used the better.
Neil
But what about 2, 3 or more wires? Feed from the same point and parallel, with a length of 3 meters. Ofcourse the capicitance increase but how much??
Rob Veld
Neil, I am guessing that the "RF current transformer" in your experiment was self-made(?)
In the 3rd paragraph, Neil, is the "series inductor of 150 uH" one and the same as the loading coil(?)
And hello to Rob Veld! Rob, your description of several wires in parallel might be one and the same as a "cage monopole" type antenna, and I place my guess that the result would be the same lowering of impedance as with Neil's copper pipe.
Rob,
I suppose using multiple wires would have a similar effect on this. I also guess that there is a diminishing return to increasing the diameter but for now I don't plan to do more experiments. A good web search would probably yield such information but I needed the numbers specific to my antenna so I did the test.
Carl,
The RF transformer is home brew and was described a few year ago here. The inductor I used is one I had wound for another purpose and it was handy for use in this experiment.
Neil
A 2" galvanized pipe?
I once made a 50 foot vertical for Ham radio use out of aluminum down spout pipe.
Worked fairly well.
Aluminum downspout pipe is made of thin aluminum, so I think of red glow if enough Watts are injected into it.
55 gallon drums welded end to end would be fun to try.
Neil
I have always wondered (even as a small child) whether 55-gallon drums stacked to a height of 30-feet and filled with mud would actually provide an elevated ground that Rich would accept?
How about 3 meters high and have water in them and then somehow attach a wire to that and see what would happen. Probably not much. I know copper makes a great conductor. As far as using 55 gallon drums for an antenna? Probably would make a better ground than anything else.
Thats pretty much what an AM tower is. They would likely make excellent antennas until they rusted out.
What about a really thick copper post? Get one 3 meters but say 8 inches around. If there truly is not rule about how round the antenna is that too could be a benefit to the part 15 station at the same time.
Read back to one of Neil's earlier posts, TheLegacy, where he says: "I also guess that there is a diminishing return to increasing the diameter."
And then Neil said he's going to do more tests, so we'll find out more in the future.
An 8" diameter copper pipe would be very very expensive.
I also heard somewhere that a wider diameter antenna increases the bandwidth, and I think that directly determines the audio frequency response that goes out. Is that still true?
LOL! This topic of 55 gallon drums reminds of a guy who I knew back in 1969 who actually attempted to make a spaceship or something similar to what the apollo space mission rockets looked like. He welded 3 55 gallon drums one on top of the other at the rims. He said this was a 3 stage rocket for a misson to the moon. Long story short, he wasn't all there, that thing was never launched and have no clue what happened to it. He was admitted before it was finished. True story.
Bruce.
MrBruce go scout for that rocket can and drag it home to do antenna experiments with it. You'd have the best Part 15 antenna in the nation.