TheRadioBoard

Do go on - that's something I haven't heard of or considered yet. This basically means the presence of the power lines might effectively reduce my ERP, in the same way that a steel reinforced building or a balcony railing would?

They appear to be close to the near field of an HF antenna. Power lines are essentially at rf ground. You see where this is going. They probably won't have a huge impact, but they'll soak up some rf. They could act as reflectors, too, with some effect on antenna directivity. But with all the close-in metal you have to deal with, you probably couldn't measure the difference.

73,

_________________

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

And yet another glowing reference to the strip-based magnetic loop design, where Dan Tayloe weighs in:

http://mail.qrp-l.org/pipermail/qrp-l_q ... 05527.html


http://www.azscqrpions.org/ScQRPions_Wi ... Report.htm

Earliest reference i can find to the idea is 1998:
http://groups.google.com/group/rec.radi ... 01c21f2d6/

I'm thinking now about how to mount my foil loop so it doesn't deform (and change resonant frequency) when a slight breeze blows. A stiff frame with the foil tightly stretched around would do, but I think there's a simpler way still (if I make the loop slightly smaller and give up 40m efficiency). Stay tuned.

This software looks interesting for fullly 3D electromagnetic simulation using the FDTD method: http://openems.de/index.php/Main_Page

I fed a bit of RF into the loop and couldn't get SWR less than 4 bars on my rig's meter, which means unbelievably high SWR (much more than 2.5:1). The current loop configuration is that the foil, inside a thin polyethylene sleeve, is taped on the inside of a hollow box 0.85m by 0.85m, with the tails of the foil hanging down loose from the top of the box. The coupling loop is at the bottom of the large loop.

It seems it's too difficult (imprecise) in practice to be moving the foil around and adjusting the coupling loop size to try to lower the SWR. We have two parameters to adjust (the foil capacitor and the coupling loop size), only an imprecise means of adjusting those parameters, but a very narrow bandwidth that requires precise tune-up. This combination of properties makes tuning up very difficult. So the mechanical stability seems to be the Achilles heel of a foil-based loop, at least the way I've constructed it.

One solution might be to solder the aluminum foil to a thin copper tube, then solder that tube to a standard variable capacitor to allow for more precise capacitance adjustment. But that introduces loss through a solder connection and through the current path to the capacitor.

For now, I think I will suspend the foil loop idea, and try a shortened dipole, using a thick aluminum radiator with large capacity hats and high-Q loading inductors at the ends. A low dipole (6m off ground) won't have a good radiation pattern, but at least it should be more mechanically stable, more forgiving of resistive loss, and wider bandwidth than the magnetic loop.

Inductive end loading capacitance hats ... here you go, back into mechanical Hell. That antenna may have wider bandwidth than a loop, but it will still be narrow and hard to isolate from nearby conductors.


Suggestion: Try a helically wound dipole. We had some success with helically wound Yagis at ARRL in the 70s, though on the low bands. No reason it won't work on 20 or 15. You can use white PVC about 1/8 wavelength for each element and start with a bit more than 1/4 wavelength of wire. Prune to about 1.5 SWR at desired frequency. On 40 and 80 meters, capacitance hats made for slightly shorter elements, but these were big antennas.


73,

_________________

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

I see your point about mechanical stability with large capacitance hats. The design I had in mind (an N3OX design) requires six two-foot-long spokes as capacitance hats, meaning capacitance hats with 4 feet radius. Not trivial to support stably.

I'll look into helical dipoles. In fact, maybe I should try a simple wire dipole for 10m (which can barely fit in my balcony) and just see if I can trim it for low SWR, or not. I've not actually heard much traffic on 10m here - it seems to be polluted with noise from consumer electronics - but maybe a dipole will let me hear more.

I'm also mulling about if I can build a mechanically stable magnetic loop but with more loss (smaller loop diameter, smaller conductor, solder connections, rotary wiper in the variable capacitor). At 10m or 15m, the added loss resistance might be tolerable (since the loop radiation resistance is higher) and that would allow me to see if a loop can be tuned up on my balcony all, or if the surrounding metal makes it completely hopeless.

More progress on the strip loop.

Current configuration: 50mm-wide, 0.03mm-thick, 5m-long copper tape is formed into a approximately square loop, suspended at the top two corners with clothespins on a clothesline, and with a wooden dowel spreader to keep the bottom of the loop flat. Loop ends are soldered to a 50pF butterfly cap, at the loop bottom. Coupling loop (1m-circumference) is affixed to the loop top with clothespins, and is squashed into an oval for minimum SWR. Affixing the coupling loop at the top (instead of the bottom) is mechanically easier as it allows gravity to pull down the coax cable and the coupling loop, as opposed to requiring some sort of frame to prop up the coupling loop as would be needed if it were at the bottom.

Results: The loop tunes from 20m to 15m. On 20m, when running up to 2.5W, I can get the SWR to the "no bars (low SWR)" reading just by peaking the received noise with the butterfly cap. However increasing power to 5W increases SWR to two bars. No visible arcing is occurring.

On 15m even with low power I can't get the SWR below 3 bars. The loop circumference is 0.3 wavelengths at 15m, and the loop is approaching self resonance, so maybe the coupling loop can't effect an appropriate impedance transformation any more.

The loop can swing in the wind. Yanking on the coax cable during key down, to force the antenna to swing, reveals a fluctuating SWR as the loop is moved around and detuned. I should have expected this, as a narrow-bandwidth loop antenna will be sensitive to any change in shape or environment.

I'm thinking I need to tie down the lower two corners of the loop to prevent swinging in the wind. This unfortunately adds to the set-up and tear-down time.

Summarizing my experience so far (with the 50mm-wide copper strip and overlapping tails for the capacitor, with the 45cm-wide aluminum foil strip and overlapping tails, and with the current 50mm-wide copper strip plus butterfly cap), it seems the biggest challenge for a strip loop is the mechanical stability, which due to the loop's narrow-band nature is important to prevent detuning. Stability means stability of the inductance (loop shape and loop position), stability of the capacitance (overlapping tails need somehow to be finely adjustable, or else the use of an inherently more stable commercial variable cap is needed), and stability of the coupling loop (shape and position; top suspension seems easiest).

One question: Why would SWR increase with 5W out (2 bars SWR) when there is very low SWR with 2.5W out (no bars SWR)? The only parameter changed was the power out (loop tuning, coupling, and position were unchanged).

Try isolating the feedline at the transmitter with ferrite cores/beads, or make an rf choke by coiling up the coax, if you can spare the length. Chances are at higher power levels there's enough rf on the outside of the coax and thus on the chassis of the rig to throw off the SWR-measuring circuit.

73,

_________________

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

After getting fed up with the finicky SWR and super-narrow-bandedness of the strip loop, I strung up my first dipole just now, for 10m, which barely fits on my balcony. Unfortunately it's resonant too high, around 29.3 MHz, but from 29.3 to 29.7 MHz the SWR is flat with no bars SWR showing on my rig even at max power out (5W). That's 400 kHz of no-tune, low-SWR bandwidth - a very welcome change from the finicky magnetic loop antenna. I also noticed how moving the ends of the dipole around substantially changed the SWR. As expected, keeping the ends in the air and clear of metal gave the lowest SWR. And at 10m, the dipole height (6m) is even a half wavelength above ground, for a theoretically good, low takeoff angle.

I think I'll seriously start to investigate short dipoles and helical dipoles.

That is a good decision! I have a short (a little over 1/10th wave length) dipole for 20 meters in my balcony, and I have worked over 40 countries/500+ QSO's with just about 30 watts max. power. It beats up all the mobile/balcony HF-antennas I have ever had.

http://www.k7mem.150m.com/Electronic_Notebook/antennas/shortant.html

Okay, but remember my mantra: There's no free lunch. You're going to lose some bandwidth when you shorten the antenna (though probably not as much as with a small loop), and you'll still have to deal with "end effects." The ends of a dipole are very high impedance, and are greatly influenced by nearby objects. If a 10-M dipole just fits, imagine how nicely a 2-M or 70-cm antenna would fit.

73,

_________________

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

Could you give some more details about your 1/10th-wavelength dipole? Are you using capacity hats to widen the bandwidth? What kind of wire are you using for the loading coils and the radiating elements? What sort of bandwidth are you getting?

Following up on a previous post...



Since I recently started to use my 100W iron, I decided to try and see if it would have enough heat to solder the shaft of this non-butterfly variable cap, and if the shaft itself was solderable. Result: the shaft is solderable! So it should be possible to use this cap without passing any current through the wiper contact, almost like a butterfly cap.

I'm a little tired of working on the magloop project right now, but when I resume it, this capacitor will probably play a role.

No capacity hats. It is just a simple dipole with coils. Wire used for coils is 0.5mm enameled copper wire and coil former is 32mm pvc pipe. Elements are bare solid copper wire, 1.5mm^2 (common stuff for a house wiring). Balun is 1:1 diamond BU-50. It is all really very simple, and can be done in one hour or so. And if you build your own balun, it is also very inexpensive.

I would say bandwidth is something like 80k with <2:1 swr. But remember, my antenna is very short. Make yours as long as you can. And you can also make it as an inverted-v!

You can use this page to calculate your own antenna: http://www.k7mem.150m.com/Electronic_Notebook/antennas/shortant.html You will get very close, just allow some extra wire if you need to make it shorter (just like you would do with normal dipole).

KR1S has a great point on VHF/UHF antennas. Working satellites from balcony might be very fun.