TheRadioBoard

Radiated power is equal to the power input minus losses, regardless of their source.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

The proper thing to do then would be to measure the losses. For measuring resistive loss in magnetic loops, I believe some people use custom-built milliohmmeters. But how about for measuring capacitor dielectric loss? How would one go about that? Equivalent series resistance measurement? Network analyzer?

I just read an interesting statement about butterfly capacitors vs. split stator capacitors. Apparently in a split stator capacitor, the RF current flows through the vanes of one stator, through the shaft, and to the other stator. The shaft causes loss. However in a butterfly cap, apparently no current flows through the shaft - only through the vanes. Is that right?

Sounds correct. All rotor vanes should be at the same potential so no current flow along the shaft itself.

Good point. It should make for slightly higher Q than an equivalent split stator arrangement. Only seem to make them in small values or transmitter size though.

http://www.ebay.com.au/itm/83-B-W-1807- ... 3371a652ae

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Got a preliminary version of the magloop built, for receiving only for now. It's 5m of copper foil taped to 5m of slightly-wider barrier tape.


(Coupling loop not shown)

5m is barely manageable size-wise for a portable antenna; any larger and it would have to be a permanent installation.

The capacitor is formed by taping the two loop ends together with polyethylene in between. The plate spacing is very small, so I need a thicker dielectric for transmitting.



By adjusting the amount of plate overlap and tweaking spacing of the 2 open ends of the loop, I can get a definite peak up to around 10 MHz. It's very easy to find the peak, as the noise really comes up. The capacitor assembly is flimsy now and prone to change capacitance, but it does work.

The receive performance of the antenna is directional, as expected for a loop.

There appears to be too much distributed capacitance to get a peak at 21 Mhz, even with the capacitor plates barely overlapping. I'll make a shorter loop for higher frequencies.

Considering the well-known metric for antenna performance, Total Copper Content, I'm hopeful that this antenna will work at least as well as a balcony wire antenna for transmitting purposes.

Just had a thought, that if you are still thinking of squashing the loop to tune it, you will also be reducing the capture area, resulting in decreasing receive levels?

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Good point about reducing capture area. I had originally thought of some sort of a rotating bar that would impinge upon one of the diagonal edges of the loop resulting in effective squashing and inductance variance, but now with the full size of the antenna apparent to me, building such a squashing mechanism looks like more trouble than I had originally thought.

My success in peaking the receive noise with the taped-together-capacitor with loose ends (acting as a book capacitor) lead me to believe that with a little more tape, the capacitor may be stable enough for transmitting. The loose ends of the loop are stiff enough to hold their shape when deformed slightly. I may bend the ends to spread outwards then use some sort of an adjustable vise-like arrangement with minimal contact area to lightly press the loose ends together.

For the n00b interested in radio, is "total copper content" a metric for receive, transmit, or both?

It's actually something of a joke. It just a general principle that for any given antenna, increasing the amount of copper (i.e. wire or radiator) will increase the antenna performance. So longer is generally better, as is capturing more area in a loop antenna. I would say that generally holds for both receive and transmit, but antennas are so fiendishly complex that I hesitate even to make that general statement (e.g. for receiving, S/N ratio is more important than raw signal level; for transmitting, impedances must be carefully matched - so more wire isn't always better).

More detailed explanations from more knowledgeable folks would be welcomed.

As qrp-gaijin (Translation: guy whose landlord is going to throw him out for building a spaceship in his living room) noted, it's a joke. Antennas are bidirectional. If you improve the ability of an antenna to intercept signals, it's axiomatic that you also improve its ability to radiate signals. There are, however, antennas that perform well in one direction but might or do not do well going the other way. You'll eventually encounter them in your studies and I don't want to spoil the plot.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

But as you see from the photos, this design is lightweight, self-supporting, and collapsible - so if worst comes to worst, I can always operate from a park bench!

The barrier tape is a bit flashier than I has originally planned, but it was the only suitably-sized tape I was able to find, so I bought it just to be able to finally make some progress on this project (which has been in planning for a long time). Maybe I'll paint it black, though that would increase the weight.

I understood that part, heh heh.



I understand that as well, but if more copper = more better, then it would stand to reason that a certain length (let's say 50 feet) of 12 gauge wire will "receive better" than 50 feet of 24 gauge wire because there is more copper in the thicker wire. Is this reasoning correct?

The reason I ask is that I have a length of about 50 feet (more or less) of thin varnished wire (24 or 26 gauge, I don't recall) in my attic as a random-length wire for a previous crystal set project with my older kid. I am considering "upgrading" it however I have a safety concern due to the fact that in the summer months thunderstorms are very common in Maryland. Also, my wife will not let me hang any "ugly wire things or antennae" in the yard. So my options are a bit limited (but not quite as much as qrp-gainjin's.

I was in Home Depot the other day and noted that I can buy 50 feet of 7-conductor wire for about 22 bux. Each conductor is fairly thin-gauge, and I could short every other end on each side to make a "folded" 350-foot antenna that would presumably "perform better" than the dinky thing that is up there right now ... although as I mentioned I am a bit concerned about safety.

So ... as you can see I have a lot of questions.

Any help, advice, or technical info would be great.

The short answer is "no." And multiconductor wire arranged as you describe, would still look like a single conductor because the individual wires are so close together. For your purposes, heavier wire is better because it's stronger, but otherwise you'd see no difference. The insulation causes no observable loss in signal strength.

Hint: Look on eBay for house wire, 12 or 14 gauge. I've bought a few 500-foot spools of 14-gauge for less than Home Depot/Lowe's prices, even with shipping added. The insulation adds a little dielectric loading to the antenna (makes the antenna seem slightly longer than it is); when cutting an antenna to be resonant at a particular frequency, the insulation is a factor. Otherwise, other than the fact that it adds weight, it isn't.

Proximity to the ground also makes an antenna seem longer, so you never can cut a resonant antenna on the ground, haul it up, and expect it to hit the sweet spot. There's always some pruning, cutting 6 inches or so from each end until you get there. Antenna equations are correct for antennas in "free space." The ground, trees, and other nearby objects always interact, so the equations are only a close approximation. Right now you're building random-length antennas, so you don't have to worry about the equations. Long, high, and in the clear are what you're after.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

Good stuff, thanks!!

It occurred to me that with appropriately-shaped plates it should be possible linearize the angle-vs-capacitance function of a book capacitor. With little metal plate material near the hinge and increasing amounts of metal away from the hinge, the huge amount of capacitance change at low angles should be reduced. The shape of the plate would look something like a thin line of metal extending away from the hinge then flaring out wider as the distance from the hinge increases. For mechanical stability the oddly-shaped metal plate would need to be mounted on some solid backing material (e.g. a copper foil layer on a PCB board, or copper tape on a polyethylene slab).