TheRadioBoard

I'm considering a QRP amateur transmit/receive antenna that can fit on my balcony (2nd floor), and it looks like I can swing a full-wavelength vertically-oriented quad loop for 10m (2.5m wide x 2.5m high) and 15m (5m wide x 2.5m high). I understand this antenna requires a quarter-wave 75-ohm coaxial matching stub to match it to a 50 ohm rig. However, having no experience with this antenna, I'd like to try it on receive first before I go out and buy a couple meters of 75-ohm coax for the matching stubs. Also, I don't yet have an antenna analyzer or noise bridge, so even if I had the coax to make the stubs, I still don't yet have a way to measure antenna impedance in order to trim the matching section properly.

My question is, will the receive performance be seriously degraded or distorted if I connect the quad feedpoint directly to 50 ohm coax and run it into my 50-ohm rig, omiting the usually-required quarter-wavelength 75-ohm matching section? In particular I'm concerned about noise pickup and if it will be worsened by an impedance mismatch, or of the noise level isn't really affected by impedance mismatches.

I'm also thinking it might be possible to use a homerew T-tuner to tune up the 15m loop on 20m. Any idea if this will be possible and reasonably efficient? Can antenna modeling software (like the often-used EZNEC) simulate the effects of lossy matching networks? Or could that be done in QUCS?

Thanks for any advice.

You needn't bother. In free space, a full-wavelength loop (circumference = 1005/f MHz) has an impedance of 200 ohms. At QRP power levels a simple 4:1 impedance ratio transformer wound on an iron-powder core will work to match into 52-ohm coaxial cable. The turns ratio (primary being the radio side) is 1:2. A T68-2 core works at 20 M. Never tried one for 10 M, but my sense is a -2 would still work there. Actually, a T50-size core should handle 5 W but you are going to crank that 817 up to its full 10 W, so use a larger core to prevent saturation. EDIT: Don't even think about using a ferrite core for this application.

You won't be putting this antenna in an ideal location if it's close to the building, but doesn't the 817 have an internal antenna matching module? The noise pickup for the length of coax you'll use won't be much of a problem if you aren't plagued by noise now. The transmitter will automatically reduce its power output if the SWR gets too high, and that will be that.

Don't get cheap and try to tune a loop for one band on another band. The bandwidth over which you can match will be narrow and you'll spend more time adjusting the matching network than operating. Pick one band and stick to it while you learn operating practices. You'll make plenty of contacts on 10 M.

73,

_________________

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

The feedpoint impedance for a full wave loop is about 100 ohms. I have 2 full wave loops in use, one for 40 and one for 10m. I use a Q section of 75 ohm tv coax (rg59). For just receiving purposes, I wouldn't worry about the Q section or a balun, it isn't that big of a mismatch. Sure it would be better if you do use the Q section, but you could get started without it. When you're ready to use it for transmitting, then you would need to go back and add the Q section.

What makes loops so good, is that they have a gain of 2.1 db over a dipole. Loops are also more quiet on receive over dipoles and it helps if you have a lot of man-made noise in your area. They have a very wide band width too and usually will cover the entire band without a tuner. If you make a loop for say 40m, then you can tune it for bands higher than 40m, but not lower. As soon as I'm able, I'm planning on a 3rd full wave delta loop for 80m.

I prefer the delta configuration for my loops and alway feed them at the bottom corner with the center coax conductor leading to the vertical part of the delta. That will make it vertically polarized and it will have a lower angle of take off, which will give you farther bounces. By the way, I do have an MFJ 259b antenna analyzer. They sure are great for building antennas!

Here is a good web page with more information about building loops and the formulas for the loop and the Q section:
http://www.iw5edi.com/ham-radio/?full-wave-loop-antenna,164

_________________
73, Bill NT9K
http://www.nt9k.com

If you’re the smartest one in your group, your group is too small..

Thanks for the responses guys. Sounds like a 10m full-wave loop will be quiet, have gain and wide bandwidth, and fit on my balcony. That's a pretty darn good combination. Let's hope propagation conditons cooperate. This weekend I'll try to whip up the loop and a toroidal (iron powder, not ferrite) transformer on a T50-6 core I have lying around.


Unfortunately no. There's an Icom I think with similar specs to the FT817 that has a built-in tuner, but the Icom is larger and has less coverage than the FT817.


How about for wide-band receiving - would it make sense to try and tune a 10m loop for reception on 20m or even 40m?

The reason I ask is that indoor random wire antennas (about 2-3m) at my location connected to the FT-817 are incredbily noisy (I saw S8 and above at some frequencies). I can't receive anything with random wires. On the other hand, when I connect my two-turn, shielded, small loop antenna cut for 40m and 80m (http://web.archive.org/web/200608051813 ... gloop.html), the receiver goes blissfully silent and I can hear acceptably well on 40m and 80m - lots of SWBC and CW. However I have never heard anything on the FT-817 above about 10 MHz when using the loop - in particular, 20m always seems dead though I know there must be hams on that band when I can hear 40m hams. So, for reasons not apparent to me at this stage of my antenna study, my shielded loop is very quiet but can't receive 20m and up, while random wires are very noisy.

Thus my desire to find a good wideband receiving antenna. Can a 10m full-wave loop be tuned for good reception on other bands? If so, would something like a T-tuner be good for this?


Sounds like good advice. The seductive thing about the FT-817 is its offering of all modes and all bands in a hand sized package. That makes it more than a little tempting to try to make an all-band antenna system as well. I think remotely-tunable magnetic loops could possibly be a good all-band antenna, and hope to investigate them more later.

One more question about operating QRP (5w or less) in the field. Let's say I have a wire antenna (dipole, quad loop, end-fed-halfwave) cut for resonance that works at home. If I take that into the field, the environment and probably the resonant frequency will change. Is this something to worry about in practice (requiring a tuner and maybe some kind of antenna analysis instrument), or does one generally just deploy home-cut wire antennas as-is in the field?

It's interesting that you don't hear 20-M signals. Sounds like your apartment is acting like a waveguide beyond cutoff, blocking signals below a certain frequency. By the way, Bill was right and I can't believe I keep screwing up the impedance of a loop. It's 115 ohms, but 100 is close enough. I'll leave it to you to work out the turns ratio. I think -6 cores like lower frequencies but it can't hurt to try one.

Trying to tune a short loop (10 M) on 20 M may improve reception, but I can't imagine it will make a great difference. I hate to be a wet blanket but, again, at QRP power levels, compromise antennas are a shortcut to aggravation. Your experience with random wires pretty much rules out a vertical on the balcony, so at least you've avoided that trap.

I've seen loaded loop antennas, and loops bent inwards on the sides to reduce overall size but maintain element length. You'd need a frame, and a 20-M loop will still be pretty big. You could try a 2-turn loop, spacing the turns a couple of inches, but it will be mechanically hard to adjust the length. It will have a lower impedance, too, so plan for that.

I have a SWR bridge that reads 2 W full scale, and I take it with me when I'm going to put up a new field antenna. Nearby objects -- and there are always nearby objects when operating afield -- will affect element lengths to some degree. I can get away with a fair amount of mismatch if I use the KX1's tuner, but I like to avoid it if possible. Make your antennas a few inches long and prune them (I assume the 817 has a SWR meter function). Elecraft makes a dandy little tuner that fits in a pocket.

http://www.elecraft.com/T1/T1.htm

It will even interface with your radio.

You asked about a "good wideband receiving antenna." Let me put it this way: In your location, you can have a wideband antenna or you can have a good antenna. There are three ways to make a wideband antenna: Swamp the heck out of the feedpoint with resistance; use multiple elements of various lengths; or use a probe antenna with an impedance-matching FET front end. If you think you have noise problems now, try the latter! They have to be installed far from noise sources.

Back in the days of the Wild West, there was a saying, "Fear the man who has only one gun." It was expected that he was good with it, it being the only one he had. Don't get greedy. Pick a band, get on the air, get some experience. Meanwhile, study maps and tourist guides for places to operate where you can have real antennas.

73,

_________________

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

Hopefully the balcony-mounted and larger-than-my-current-antenna 10m loop will provide some copy on 20m. I'll find out this weekend.


Well, I recentlly spent about a hundred bucks on my future-proof reserve of leaded through-hole components, so I'm inclined to start using those rather than buy a ready-made tuner. Let's see... a tiny circuit with noise bridge (always inline and switchable in/out of the transmission path), a T-tuner with switchable tapped inductor and a couple of polyvaricons, and maybe a little LED SWR indicator. Should be enough to tune up a wide variety of antennas at home or in the field.

By the way, how much of my precious QRP power can I expect to lose in the tuner when I use one?

Hard to say! A lot depends on the amount of mismatch, but probably no more than a dB.

73,

_________________

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

I found another WN6F all-band coax loop design that looks promising:
Image link: http://web.archive.org/web/200511040052 ... edloop.jpg
(from http://web.archive.org/web/200608051813 ... gloop.html)

I built a 40m/80m loop according to the author's guide and it worked better than I expected (on 40m and 80m - but it seems more deaf than a random wire on the higher bands), so this all-bander might be worth a try.


I now have a strange desire to watch "Shane" again.

I rest my case!

73,

_________________

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

Made significant progress in improving the rx situation here.

First, I finally found and eliminated some local noise sources: some wall-wart AC adaptors, an AA battery charger, and an older lamp. With those unplugged/turned off, my indoor random wire, even lying on the floor, was tolerably quiet.

Next step was to listen around the bands. I could make out some 40m, 15m, and 12m CW signals pretty well, but 20m was just plain noisy, with 10m only slightly less noisy.

I decided to try a full-wave loop for 15m and cut approximately 14.45m of wire plus 1m extra. I threw it up on some hooks on the balcony and ran it straight to my receiver with no balun and no coax.

I could receive a lot more signals than I was used to with my previous 40m/80m loop. 40m was hopping with local (Japanese) CW. 20m was still just noisy and only strong signals were audible above the noise; I only heard a few CW stations. 15m and 12m provided good copy: I heard CW signals from China, Taiwan, Russia, and even one from New Jersey! 10m was also noisy and I only heard one JA station.

It's evening now so I took the antenna back inside (I was running it through the open balcony door, and the night air is chilly). Inside I can still hear some kind of pileup on 15m (though I can't hear who they're working) and 40m still has some CW activity.

So probably daytime 15m or 12m seems like a good band to start with.

Next, I need to build that noise bridge, since otherwise I have no way of properly trimming/tuning the antenna.