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[Articles Home]  [Add Article]  

Magnetic Minds Think Alike

Brian Levy (W2BRI) on December 9, 2002
View comments about this article!

With all the talk of CC&Rs and the many housing restrictions put on hams today, many are wondering if HF will only be part of their hobby when mobile and away from neighborly naysayers. I came close to this problem when I moved to a new house that had a major power line issue. My beam antenna and other toys were not going to fit -- no way, no how. To make the problem worse, I am a late night kind of guy. Which means after 10 PM, there aren't a whole lot of people around to ragchew with unless your on the low bands. I figured 75 meters was my ticket to late night ragchewing -- which got me wondering, what the heck I was going to do to get on 75.

Now I like to build antennas, and 12 years of living in New York City, the mind will churn out creative solutions to space issues. I have tried all the classic restricted space scenarios, the hidden long wire, the hamstick dipole, the shortened vertical, the indoor dipole, I even tried an EH antenna. All of these solutions worked to some degree or another, but non of them really satisfied my performance expectations.

I'll define for you what I consider to be acceptable performance and it's a very easy concept. If I call CQ, someone should come back to me within a minute or two. If I am talking to a station, I should be well heard and they should hear me well most of the time. No offense, I'm not a QRP kind of guy -- I'm too impatient. Nothing unreasonable so far. Additionally, if I am talking to more than one station on frequency, everyone should hear me and I should hear them. Now on twenty meters with my beam at 45 feet, everyone heard me and I heard them -- pretty much most of the time, space weather considering. Yes, I know you can't compare a compromised system to a directional efficient antenna. I can have high expectations, maybe sometimes too high -- I'm trying to change, really. Yet, every compromise had some factor of disappointment. I guess that's why they call it "compromise." So how was I going to get on to 75 meters with plenty of power lines, and no space for a dipole? I thought about vertical solutions too, but verticals take up space if you include the radials, and I needed something stealthy.

While reading antenna guru Tom Schiller's book, "Array of Light," he mentioned using magnetic loop antennas extensively (Tom is the owner and designer of Force 12 antennas and his call is N6BT). I then pondered and read up on magnetic loops. They seem to have so many interesting and desirable qualities: high efficiency, small footprint. I reasoned I could build a magnetic loop for 80 meters that was a total of 12 foot by 12 foot out of some pipe and it would exhibit decent performance in the 50% range, theoretically of course. So I called Tom up on the phone and asked him about the antenna. He said he would be happy to build one for me, but he said the capacitor arrangement was my problem because of the obvious cost of the capacitor. Here in lies the rub, magnetic loop antennas require a high voltage capacitor to load the loop to resonance, but that capacitor can cost upwards of five hundred dollars for a nice and new vacuum or ceramic variable. There are several other capacitive strategies that can be employed, but Tom made it clear he didn't want to deal with it. He was kind enough to give me some suggestions, which I did implement and find to be very useful when getting the loop started. I'll explain that later. I love dealing with complex and interesting challenges, and a magnetic loop is just that: complex and challenging. So after several emails between us, and a couple of months (I was in no rush), a large heavy box showed up.

Step by step I put the antenna together. Everything was well marked and it didn't take more than a half hour to assemble the loop. The loop was a large square twelve by twelve feet in size made out of two-inch aluminum pipe. Instead of using a fancy capacitor to load the darn thing up, I used some coaxial stubs at a value of about 170 pf (The coaxial stubs were Tom's idea). The feed system was a bit short for my loop, so I did some modifications and had the thing resonant after a couple hours of fidgeting with the coax and feed and some emails to my buddy Ben Smith W4KSY, a mag loop expert who was of great help when it came to getting a good match.

I waited for night to fall and turned on my rig to 75 meters. I heard several Bay area stations (I'm in Los Angeles) on 3.863 having a QSO and popped in my call. They came back and I joined the ragchew. I was pleased with my first test, everyone heard me and I heard everyone on the round table. Good signal reports all around, and I was on 75! So far so good. Now using coax as a capacitor works pretty well considering it's cheap and readily available. The major drawback is that it is fixed to a particular value and hence your resonant point is stuck on one part of the band. Also, coax can't take very high voltages, so you're stuck at about 100 watts max. Don't even think of running AM -- you'll blow up the coax and set it on fire. I learned this the hard way.

There are challenges when dealing with magnetic loops, and that's part of the fun. They have a high Q, and are therefore resonant at one spot. Now, you may have 20-30 KHZ of bandwidth to move around on, but that's it. You try to move too far from your center of resonance, and the antenna doesn't work efficiently. So I accepted the fact that I was stuck around 3.863, but I was on 75! Did I mention I was on 75 meters! Did I mention I was having fun? Night after night, 99% of the stations heard me well and I heard them. The loop was working up to my expectations. I was amazed. Imagine that, a 12 foot by 12-foot square. But I wanted more, and I started to think, if only I could change frequency, if only I could run some power.

I acquired a used Jennings vacuum variable, 5-750 pf which tested well over 12 KV. I bought a 6-RPM DC 12V motor from a surplus place here in Los Angeles, momentary push buttons, relays, and project box. Got myself a waterproof box, made some holes, ran some wiring, and poof: I had a magnetic loop that could now change frequency from the warmth and comfort of my radio shack. The loop actually works very well on 80 meters, you can find me operating nightly on 3.977 and 3.983 from about 10:30 PM Pacific Time nightly. Also, the loop works fairly well on 40 meters as well even though it exceeds the theoretical 1/4 wavelength rule.

There are a lot of factors relating to Magnetic Loops that are considered to be hard fast rules related to construction and operation. Many of them are very debatable. For instance, there are so many issues like welded joints vs. mechanical, shape (circles vs. squares), copper vs. aluminum, etc… And I cover much of this on my magnetic loop website at http://www.standpipe.com/w2bri . So please, the purpose of this article is only to introduce this antenna idea once again for those with limited space considerations. I hope that maybe more people will be inspired to enjoy this technology, which has brought me so much pleasure. I also explain exactly how to build one of these loops and how to get them to work. So enjoy and 73,

Brian Levy, W2BRI

Member Comments:
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Magnetic Minds Think Alike  
by N3HKN on December 9, 2002 Mail this to a friend!
I followed the instructions on Brian's site for a 3.5ft square loop. It works! For comparison I have a GAP challenger vertical located about 10ft from the loop.

The loop is mounted on 10FT of PVC pipe on the deck which is about 8ft above ground itself (townhouse). My unscientific findings are that the loop outperforms the vertical with USA stations that are within its lobes. However, stations broadside to it suffer many s-units of signal loss - as it should be. DX stations are about 1 s-unit down from the vertical.

The pattern of the loop is fairly even apound the loop and does not seem to have a strong low angle when mounted vertically. I have not tried it yet, but horizontal mounting should increase the low angle radiation IF mounted 1/4 wavelength high.

A construction note: I used the coax capacitor method at the top of the loop. It worked. However, at power levels oabove 50W it arced through the center conductor insulation. I was using 9913. With RG-8 it seemed better but eventually had an arc there as well. When constructing the stub the center conductor , with insulation, should extend about 2 inches or more beyond the shield. My problems occured at the edge of the shield because there were some "stray" fine shield wires extending slightly beyond the main shield body. The very small, very sharp, points did the predictable and formed "high pressure" points for the electrons and eventually spilled over in an arc. Aluminum foil solved that by wraping it around the shield and extending it beyond the shield with its end dressed in a good circle around the insulation. Recently I fiddled it and ended up with a point on the foil - arc... I used RG-8 but perhaps RG-213 would be better for the tuning stub. I have a split stator 4500v air vairiable on order. Will rotate with an old electric screwdriver.

In summary, I also have tried the EH stuff but touchy and no obvious good results. The loop worked immediately. It should be thought of as a dipole operating at the same height as the loop. Given my observations over a few weeks I believe that the patterns is sharper than the dipole and the signal strength is equal. In constrained environments it is the answer!
Dick N3HKN
 
Magnetic Minds Think Alike  
by N0AH on December 10, 2002 Mail this to a friend!
I used the AEA Isoloop for years, wore out, but worked great. I did notice a lot of fluctuations in resonant points under various wx conditions. Loop seemed very sensitive to environmental factors, heat, cold, dry, wet etc.......likely due to the flexible metal band used for the loop- These antennas create a very high and dangerous voltage and should not be in an area where they could be accidentally touched while in tx. Also, the high voltage limits reasonable power fed into the antenna, 100 watts max is a good safe limit. They also have a high Q which really limits there band width and can make them noisey on the low bands. But I feel that there is a real market for low band loops and hope Force 12 or someone would come out with a line of them. The biggest commercial reason you don't see more is due to costs of reliable motors and capacitors needed. Especially if you want to run high power. But the AEA loop went for around $500 and I would not mind paying double or triple for one put together with higher quality parts. An excellent 10-20 meter antenna and a lot of fun obviously to experiment with one on the low bands- But realize that from an RFI standpoint, they put out a spherical pattern that can get into a lot of stuff. So nieghbors beware. This is one antenna design that I used that created knocks on the door- Others claim it is RFI free, but I didn't have such good luck. Anyhow, this is an excellent article! And a loop is a great stealth antenna-
 
RE: Magnetic Minds Think Alike  
by NB6Z on December 10, 2002 Mail this to a friend!
Where as the magnetic loop is an interresting concept, I know there are easier to construct HI-Q antenna solutions for hams with limitted space. A helically wound vertical or dipole around a piece of PVC pipe will perform like a mag loop but with less construction sensitivity and complications. The mag loop is for those who like to experiment, not the best practicle solution for limitted space antenna construction.
 
RE: Magnetic Minds Think Alike  
by N3HKN on December 10, 2002 Mail this to a friend!
This helically wound device sounds like a Hamstick. In that case you need radials. This will not be a HI-Q device but a loaded shortened vertical. I am not sure that this would perform as well as a small loop where you have the dipole character of gain over a vertical in the major lobe directions. My experience vs a GAP vertical in favored directions the loop will add an S-unit or two to the signal. Similarly in broadside to the loop directions the signal is likewise attenuated.
Dick N3HKN
 
Magnetic Minds Think Alike  
by KB2VUQ on December 10, 2002 Mail this to a friend!
Brian,

I'm glad to read that your still in California
and enjoying the hobby.

When I first heard of your experiments, I tried
to send you fifty (50) feet of good quality
low-loss foam cable, in the RG8 family.

I took the 50 feet of cable down to the Post Office
and tried to mail it to you!
The Post Office sure is different down here in Florida.

Would you believe they wouldn't sell me a cardboard
box 50 feet long by 1" H x 1" W?

What a bunch of idiots!!!!!!

Maybe I'll have better luck with UPS or FEDEX.

Best 73,
Dennis


 
RE: to n3hkn Dick  
by VE2XLT on December 11, 2002 Mail this to a friend!
Dick - I've read your notes twice but could not guess out what is the Band or Freq you compare 3.5 sq feet loop to a Gap vertical ? Is it on 20 40 or 80 mtrs ???
Basically i am interested to do a home made efficient 40 mtr Monster as i owe an MFJ 1786 covering 10 to 20 and even 30 mtr with less radiation.
 
RE: Magnetic Minds Think Alike  
by KE1MB on December 11, 2002 Mail this to a friend!
stick two hamsticks end to end and you have a shortened diploe, not a vertical. You can also use a vertical and feed it with a 1:1 balun using given lengths of wire as the other end of the "dipole". It sits like an inverted V on it's side. Works great and I do not need countless radials to make it work. So the vertical can be used without taking up too much space. I am limited to one ant. only and am allowed to only mount it in one place on a fence. I make do and it works great.
 
Magnetic Minds Think Alike  
by N6VL on December 11, 2002 Mail this to a friend!
I am in a similar situation as Brian, but worse. My entire antenna farm is located on a 12 X 6 foot apartment balcony with only 12 X 3 foot exposed. It is fortunate that there are some trees hiding much of my balcony from easy view of the neighbors and the apartment management. Yet from these limited resources, I have managed to have to be active on many HF bands including 80 meters. My main motto is to never give up. I have made tremendous improvements in the past two years and certainly there are many more that can be made.

I have tried magnetic antennas here, but I finally settled on a screwdriver type mobile antenna. The magnetic antenna was the commercially available MFJ-1786. My results were mediocre. I think it was in large part due to the stucco siding in my apartment building. Having a wire mesh within the walls acts as a shield. The magnetic loop was about three feet to the closest metal, which includes stucco and gutters. That is the best I can do. I could string a wire through the trees, but the apartment management is vigilant about keeping the grounds maintained. This includes periodic tree trimming. So I strive to keep my entire antenna system on the balcony. I suspect the best test environment for a magnetic antenna is out in the clear away from obstructions.

I have had my best results with the screwdriver antenna. I believe this is due, in part, to using the stucco part of the antenna system and not as a shield. My counterpoise is about 12 X 4 foot of aluminum window screen laid on the wooden balcony deck. Believe it or not, radials did not work. Luckily there is a small chip in the stucco siding about 18 inches from the base of the antenna that I can clip onto. There is a noticeable drop in the resonant frequency on 80 and 40 meters when including the stucco in the counterpoise, indicating that it is an effective part of the antenna system.

My main operating mode is PSK31. I rarely work SSB from the apartment. This is due to its inefficiency compared with PSK31 and less RFI (less audio rectification). At night I can extend the upper whip of the screwdriver to 20 foot. This helps the lower bands greatly reducing the amount of inductance needed to achieve resonance.

Of course this thread is about magnetic loops. I personally believe they are a viable alternative to limited space operators like myself. I want to give them another chance, because I do believe they work. A serious unbiased examination on my part would be to take the MFJ loop to a park and operate out in the open and then compare it to my screwdriver mounted on the mobile. That would help me determine if the loop is compromised by the stucco siding at home.

Several years ago, in a different apartment, I built a 6 foot octagonal loop from ¾ inch copper pipe. But it was even closer to stucco siding and other metal than my current apartment. It was slighter longer than a ¼ wave on 20 meters. I had no luck whatsoever on that band. That was in the Pactor days. I managed one feeble QSO. I now get out better on 80 meters than I did on 20 meters with that loop.

I am particularly interested in hearing from people who have used magnetic loops in environments like mine close to metal. I have noticed that Brian’s 80 is 12 foot square, yet his 40 meter version is 42 inches square. Doubling the size of his 40 meter version yields a 7 foot square on 80 meters. This is a large as I can manage in my situation. Again this is 3 foot from other metal.

I live in Orange County, California about 60 miles or so from Brian. We are close enough that we could easily share notes.

73,

Steve N6VL

 
RE: Magnetic Minds Think Alike  
by N3HKN on December 12, 2002 Mail this to a friend!
I have operated my 1 meter square loop inside and it picks up 5 s-units of noise. If I place it close to the house, but on the deck noise is still high. However, 10 ft from the house, on the deck, the noise is a low as the GAP vertical. I just installed a variable capacitor (25pf to 50pf) dual section 4500v unit. With it I can cover 20m, 17m, and 15m. Will use an old electric screwdriver to drive the capacitor with a pulsed variable speed control.


If you look at the efficiency of a 1 meter loop at 20 meters it is not very good but it is quite good at 15 meters. I hope to build a larger loop for 40 meters with max efficiency. The loops will be mounted on the wooden deck railing. Luckily the deck is 10 ft from the ground.


As to the stucco I agree from my observations that more than 3ft separation is needed from other "stuff". I also operate PSK on 20m due to townhouse restrictions which are a bit more loose than others.

I will also be operating QRP CW and the narrow resonance of the loop is a significant benefit in the reduction of QRM causing overlaod in a tiny direct direct conversion receiver (Rockmite).

Dick N3HKN
 
RE: to n3hkn Dick  
by W1RFI on December 13, 2002 Mail this to a friend!
> Dick - I've read your notes twice but could not
> guess out what is the Band or Freq you compare 3.5
> sq feet loop to a Gap vertical ? Is it on 20 40 or
> 80 mtrs ???

If I recall correctly, the small loops have been advertised as having an efficiency ranging from 78% on 20 meters to 92% on 10 meters. Both seem reasonable and I obtained about the same numbers when I modeled a small loop on EZNEC.

However, the AEA and MFJ loops take great pains to keep the losses low. These very small loops have impedances of a fraction of an ohm. This means that any losses in the capacitor or matching section, even very low resistance losses, can degrade the performance dramatically. AEA and MFJ solve this with a very high-quality capacitor, a design that eliminates any soldered or mechanical connections (the capacitor is welded to the loop) and a small loop used as a matching "network" to couple energy into the loop.

Below 20 meters, even these heroic measures are not enough to give a highly efficient antenna.

Of course, any small antenna will have poor efficiency on the low bands. A typical mobile whip may have an efficiency of only a few percent on 80 meters.

73,
Ed Hare, W1RFI
 
RE: to n3hkn Dick  
by W2BRI on December 13, 2002 Mail this to a friend!
According to almost all the literature I have seen on loops, all connections must be soldered or welded. Please read on because I do not believe it's that simple. It appears that mechanical connections create resistance in the conductor which cause major losses in efficiency. It is interesting to note that all the loop articles I have seen and read do not discuss mechanically built loops, only welded versions. I myself have built both mechanical and soldered loops and have not found major variations in performance if the mechanical connections are well made. This is purely observational. My 12 foot by 12 foot loop for 80 meters has no welded or soldered connections and works well from my daily observations and anecdotal evidence. Keep in mind, however, that each segment of pipe has a 3 inch taper that slides into the next pipe (and is then bolted together) -- so there is plenty of overlap for each connection. I have also built loops mechanically, tested them for some time, and then soldered them together to see if there would be a measurable change. As of yet, as long as the mechanical connections are well made, there seems to be no measurable difference. These conclusions do not seem to correspond to the general literature of loop making, but I feel it is important to put this information out there for debate.

 
RE: to n3hkn Dick-Loop Calculator  
by N3HKN on December 13, 2002 Mail this to a friend!
I suggest to everyone that they go to the German site http://www.funklab.de/mloop.html and download Antenna Calculator 1.3 für MagLoops von KI6GD toward the bottom of the page. There you can calculate almost any reasonable loop configuration and eliminate much of the debate based upon folklore.

Validation of the calculations can be done with info from http://you.genie.co.uk/carl.littlejohns/magloop/magsoft.htm .

They work - I installed a 20pf to 50pf variable rated at 4500v in a plastic box on my 3.5 ft square loop. Comparing it to a GAP vertical I found, on 20 meters) an assortment of equal, stronger than, and weaker than signals. I took no pains in construction. Soldered the joints. Connected the capacitor with #14 wire using fricton clamps to I could quickly play around. I have had contacts where the loop was 2 s-units better than the GAP, and some 2 s-units worse. I attribute the loops better performance in the +2 units to its dipole like pattern vs the vertical that has to distribute its energy equally in a 360 degree area. Angle of arrival/takeoff may also play a part.

We can discuss this forever. Go to Brian's site and follow his directions and build the darn thing. Using coax as a capacitor (RG-213) it is 20ft of 1/2" copper pipe and some elbows with some pipe left over. Probably $20 with a new propane torch tossed in. Play with it inside but use it outside at least 10ft from the noisy house. Buy a 10ft piece of 2" PVC while you are at the store and a roll of tape. Use the tape to tie the loop to the 10ft mast to get it off the ground a respectable distance. If you have an analyzer it is a no brainer to tune. If not 5 or 6 trips to/from the shack will do it. It is winter and thus the axiom of playing with antennas in bad wx definetly makes them work better!
Dick
N3HKN
 
RE: Magnetic Minds Think Alike  
by W8JI on July 8, 2003 Mail this to a friend!
Coax makes a very poor Q capacitor, especially if the coax has any length.

Read
http://www.w8ji.com/traps.htm

I've measured coaxial cable capacitors in traps and found the Q around 600. This is among the poorest of ANY type of "capacitor".

A small dipole, properly constructed with a GOOD loading coil will also have more efficiency than a "magnetic" loop. The problem is tuning the dipole to new frequencies. Helically loaded short dipole antennas, by the nature of the poor loading Q, are generally inefficient antennas.

The small loop is only "magnetic" near the antenna, it actually is electric field dominant at a small distance away!!! I have a graph of this at:

http://www.w8ji.com/radiation%26fields.htm

All antennas couple to objects around them, and small loops are no exception. It is really a random toss of the dice if a loop has "less noise" or "more noise", or has less external loss in a given situation.

What is particularly interesting is the small loop is an electric field antenna once you are about 1/10th wl away, and a small dipole is a magnetic field dominant antenna at that distance and further!!! Yet many of the loop advantages are claimed to because it is a magnetic radiator. (Of course at a wavelength distance all small antennas are equal in field impedance).


73 Tom
 
RE: Magnetic Minds Think Alike  
by W2BRI on July 8, 2003 Mail this to a friend!
Tom,

The core issue regarding coaxial capacitors is that we do not use long lengths. With the short coax length we achieve decent performance from these loops. My first pick would not be a coaxial capacitor, as I use a vacuum variable capacitor. Yes, coaxial caps are not optimum. However, at a large cost saving, a coaxial cap will get someone on the air with good results.

In terms of your comment regarding a small dipole, what do you mean by "small" dipole? I have a friend in Queens that uses a 30 x 30 inch copper loop and is very satisfied with its performance on 20 meters. Your dipole would certainly be a lot bigger. Also, I have built small dipoles with loading coils and had very poor results. The short dipole was 3-4 s-units down. Of course varying construction will matter a lot in relation to performance, but for size, I think the loop is excellent.

There are certainly better antennas for HF than a magnetic loop, however, for their size (stressing size) and performance, they are hard to beat.

73,

Brian, W2BRI
 
Magnetic Minds Think Alike  
by KC4ZGP on February 10, 2004 Mail this to a friend!
I'm Tim in Warner Robins, Georgia. I use a circular, one-meter diameter copper tube loop for 14.07MHz at fifty watts for PSK31. It sits one meter off the ground.

The main loop, coupling loop, and capacitor are fixed to a wooden plank. Then the plank is attached to a broomstick. I have a short pipe in the ground just at ground level. And when I'm ready to PSK'it on the weekends, I poke it in the pipe, connect coax, tune, and have at it.

Since I see no PSK31 activity exept on twenty meters, and I'm not a station chaser, I've never concerned myself with multi-band operation, thus eliminated the remote tuning function issue. I'll wait for folks to roll around to twenty meters.

Loops do work!

Kraus
 
RE: Magnetic Minds Think Alike  
by N3LCW on August 12, 2008 Mail this to a friend!
One can overcome the efficiency problem of small loops by doing two things:

1) increase the loop size to 1/4 or 1/8 wave at operating frequency.

2) Feed the loop NEXT to the tuning capacitor using a matching transformer/toroid. This is the method G0CWT has found to be very effective. Refer to his website http://g0cwt.co.uk/magloops/ for details.

I have since built loops based on G0CWT's approach using the matching transformer feed point next to the tuning capacitor and the results have been overwhelmingly amazing! From my postage stamp size yard I was able to suspend a 10 foot by 22 foot loop using two camouflage tent poles each 12 feet long. The loop uses black 14 gauge wire, an 11 inch length of RG8 coax as the tuning stub, and the matching transformer described in G0CWT's web page. This loop resonates on the 75M net frequency I check into on weekend mornings. I receive constant praise for the signal strength this loop provides with my 100 watts of applied power.

Using the 9:2 ratio matching transformer G0CWT describes I have loaded this loop on 160M with yet still amazing results, getting S9 and 10/9 reports from the midwest. A new tuning stub was cut for operation on 160M.

I have used a 40M and 20M version of the G0CWT loop with equally impressive results.

The are not compact loops but they are very small compared to a full size dipole and will fit in small yards as mine without drawing any attention from neighbors. The 10ft by 22ft loop will certainly allow many amateurs the chance to operate on 160M with a very effective signal compared to short lossy loaded verticals. The use of wire makes the construction of these loops simpler and less expensive than working with copper tubing.

I will be adding the results of my loop experiments to my web page on QRZ.com soon.

The two things I recommend mostly before anyone starts working with loops is an analyzer like the MFJ-259 and also a good capacitance/inductance meter. This makes design and tuning much easier.

Andrew N3LCW
 
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