Fun with external loop antennas - Printable Version
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Fun with external loop antennas - BDM - 12-21-2011
When I finally got around to building my first one. With a little help from Xray, she really works well. I'm really impressed with it's performance. I have to say it rivals my 160' long-wire on the BCB depending. Plus it really cuts the neighbor induced noise, which is an issue around me
Cost = $15. I'm counting the wire used along with other small odds and ends. I stripped the tuner from one of my junk AA5 sets. It measured about 10 - 467 pf on the tuner. So I used 15 winds for the main loop, and one wind for the sense loop. Connected the sense loop to A/G and the tuner to the main loop (for those that may not know already). I'm sure this is old hat for some of you, but new for me
I tweaked it some with a spacing of .100 between the 15 wind strands. I ended up bringing them together butt up against each other, but noticed no real difference except it now tunes below 500/kHz. I'll probably play around a little with the spacing to see what it does above and below. Anyhow it's no museum piece like some of them I've seen. But it tunes the whole band very well. I stuck it on a lazy-Susan so I can easily turn it for direction. Specs are;
One Philco 37-630 (Philco not required but preferred
)18" x 18"
10-467/pf tuner
24 gauge magnetic wire
15 turns (spacing can vary)
1 turn sense loop spaced 1" below main loop.
Some of you guys should consider trying one, especially if you lack space for a long wire. Fun to build and the performance is really amazing. I'm planning on another I'll slap together this weekend. Working out the specs now. So do any of you fine gents have or have built loops? If so show us what you have. Tell me you most guarded secrets to a well built loop
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Re: Fun with external loop antennas - codefox1 - 12-22-2011
Yes, amazing those loops. I used an old 4 ganger variable cap with switches to make channel tuning a little bit easier. Can clock in a mica or two to make your nut too.
Re: Fun with external loop antennas - BDM - 12-25-2011
Built this 30" x 30" and learned a few things. This one performs better. More so on peaks & nulls. Seems to have a higher Q. I built it beefier than it needed to be. So learn as you go, make them lighter
Note from site admin: Sorry, but the photo which was attached to this post is no longer available.
Re: Fun with external loop antennas - TA Forbes - 12-26-2011
How do you have the main loop, sense loop, and variable cap connected? Are the 3 gangs wired in series?
Re: Fun with external loop antennas - codefox1 - 12-26-2011
The main loop is wired in parallel with the variable capacitor. One section 10-365 is good enough for most of the AM band, but an additional section added will tune lower. You can use a toggle switch to add or subtract sections fixed capacitors to make use of what you have on hand. The sensing loop is used just to connect to the Antenna and Ground terminals of the radio and is not connected to the main loop. If the radio is in close proximity and has an internal ferrite or loop antenna no direct connection is required, so in that case the sensing loop is not used or needed.
You tune the radio, then the loop's capacitor, and back and forth to get the best reception on any given channel. Note this arrangement is highly directional, so you can indeed get two stations on one channel, sometimes both are readable.
Re: Fun with external loop antennas - BDM - 12-26-2011
[Video: http://www.youtube.com/watch?v=b61VRw1KqxI]
Re: Fun with external loop antennas - gvel - 12-26-2011
Nice video, Brian! It's amazing how directional that is.
Not only does it null out 'JR's bleed over from 760 to 750, it also nulls out clear channel 750, WSB, Atlanta, when you "point it" east!
Re: Fun with external loop antennas - BDM - 12-26-2011
Yes they are. With all the prior post I've read in all forums about noise, or "my 20' of wire isn't good enough", etc. etc. I'm not sure why more collectors do not build these. I have seen guys complain about having to re-tune the antenna everytime that band cruise. H**l that's part of the fun
I just recently started building them and love it. I'm on my fourth one now and just experimenting. Next of the list is a spiral version. I really like the look of those.
Re: Fun with external loop antennas - TA Forbes - 12-26-2011
I'm sold! I happen to have a variable cap that I harvested from a Crosley 25AU that should be perfect for this!
Also have a bad filter choke that I saved for the wire!
Re: Fun with external loop antennas - BDM - 12-26-2011
I use this calculator. Saves from doing the math yourself which isn't hard. But you'll find this calculator isn't exact. I always use it and error on the side of "too much". Makes it easy to trim back to fine tune for antenna.
http://www.angelfire.com/mb/amandx/loop.html
Re: Fun with external loop antennas - TA Forbes - 12-26-2011
I saved the link to the calculator and loop antenna page. Outstanding!
It seems to me that you could use switches not only for including additional gangs but to also include (or delete) additional windings on your antenna. This would allow you to increase, narrow, or completely change the bandwidth that you intend to recieve.
I like DXing shortwave stations. How well would these loop antennas work for them? In my former life antenna polarization was always a consideration in addition to several other factors. But we transmitted and recieved, and the power available for transmitting from a team in the field was only in the vicinity of 20 watts.
Re: Fun with external loop antennas - BDM - 12-26-2011
Yes loops can work on SW. Mine actually works on SW, but it's extremely inefficient. Completely wrong inductance. The idea of bringing in further capacitance and switching wires is a good idea. I've seen a few web pages where this has been done. One HAM out there really built a monster loop. with receive amps and various tuning gangs etc. etc. for various bands.
Plan it out and post about it. I'd be interested
Re: Fun with external loop antennas - gvel - 12-28-2011
TA Forbes Wrote:In my former life antenna polarization was always a consideration in addition to several other factors. But we transmitted and recieved, and the power available for transmitting from a team in the field was only in the vicinity of 20 watts.
As a signal travels, bouncing off different levels of the atmosphere and objects at the surface, polarity changes to something other than the source.
I read somewhere, "we" receive a circular polarized wave. I do find that difficult to believe - however - since the symmetry of a circle would be very difficult to maintain. So...I think of it as rotational. The electric field changes in direction and intensity over time. This could be one reason for fading?
For line of site communications, throw all of this out the window. Polarity is very important.
In short, for dxing on the shortwave bands, the polarity of a receiving antenna is not a factor.
Re: Fun with external loop antennas - BDM - 01-02-2012
Here is the latest creation. 25" x 25". Works quite well. Nice peaks across the entire band and good nulls.
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Re: Fun with external loop antennas - TA Forbes - 01-02-2012
Agree. We were transmitting and receiving shortwave sky wave propagation of varying range. For LOS propagation, which for us was VHF and UHF frequencies, we used for the most part the antenna systems that were included with the radio. Sometimes, when it was practical, we constructed antennas that maximized both LOS sky and ground wave propagation. In arid environments, this was important, as the ground did not conduct well. For standard military VHF man packed radios, about 7 kilometers was the best you could rely on. Later, we had UHF radios that could reliably be used for 10 kilometers. There were times, when we were experimenting with these sets, when we communicated more than 20 kilometers, this was in mountainous terrain with both stations significantly elevated. If you had an aerial relay platform in support of your operation, your VHF and UHF range was dramatically increased. Usually these were MH-53s, MC-130s or AC-130s.
For shortwave comm we almost always constructed our own antenna systems. For relatively close (10 to ~300 miles) communication we used what is called NVIS (near vertical incident sky wave) antennas. They tended to be omnidirectional but were effective. This was especially good when communicating with a ship that was loitering 100 or so miles off shore, as the omnidirectional radiating property of our antenna went well for a ship that was constantly moving in a preplanned area.
For longer range comm I preferred either a sloping wire or a sloping V antenna, as it was fairly simple to determine the orientation and takeoff angle, and was easy to directionalize. I was able to transmit reports from the carribean and south/central america with a 20 watt radio using these systems. I did not like using a dipole because the lobes radiating from the antenna and the takeoff angle were difficult to control, as they were determined to a large part by the height of the dipole from the ground. In a clandestine situation constructing a "high" antenna was not practical.
Later in my career we received satellite communications sets (SATCOM) that made all the difference. They were smaller, lighter, had superior range and crystal clear sound quality. It was also at this time that we received digital equipment that allowed us to transmit reports and photographs in a digitalized "burst" transmission. The "burst" took less than a second, which reduced transmitting time and therefore prevented opportunities for the "bad guys" to direction find our station.
The circular waves you speak of may be the "lobes" that are radiated from all antennas. The shape, direction, and coverage of the "lobes" depend on the specific antenna. Although the lobes are "circular" in a 3-dimensional sense, the waves present in them are not.