[Lowfer] Antenna Questions

Tue Mar 3 21:53:32 EST 2009

Hi Jim,

Welcome into the world of Lowfering! I'll answer some of your loop questions on this round and let others cover your vertical questions. 

First, don't take this too negatively, but I'm not so fond of your "tinkering" approach in achieving a sound antenna design. Based on about 10 years of LF antenna design work, I have found that there are far too many variables to handle without a theoretical approach you may end up spending a lifetime without learning anything about how to optimize your design and make it match the unique set of variables at your home base.

Here are my comments and suggestions on TX loops:

1. Invest in a compound bow having a pullback of at least 50lbs so you can place a leader line well over to top of your trees. Ebay is a good source. The trees, themselves, offer an excellent support system, and only a minimal amount of supporting devices or ropes are needed.

2. Study my two articles on loop design particularly the second one. Gain an understanding of the factors that effect the loss resistance of the loop conductor (Rac). Realize, also, that maximizing the loop area for a given configuration is extremely important.

3. Added inductance to the loop conductor will have nothing to offer except increased losses. 

4. Always set up the loop in the vertical plane.

5. Start out with a #10 or #12 conductor to get a 'feel' for resonating the loop and matching it to the final. This loop WILL get out with its ~1A of current if you have done your homework.

6. Two antennas resonated at the same frequency and located close to each other will interact. Open-circuit or short-circuit the unused one when it is not in use.

7. You will find that ground losses will not be too severe if you keep the lower horizontal leg at least 6ft above ground. This assumes average soil conductivity.

8. You just might find that Part 15 experimentation is more fun than Part 5.

Good luck! We will be looking for you.

Bill Ashlock

> From: w4jbm at bellsouth.net
> To: lowfer at mailman.qth.net
> Date: Tue, 3 Mar 2009 19:49:39 -0500
> Subject: [Lowfer] Antenna Questions
> 
> I'm hoping to have a LowFER beacon on from West Georgia in a couple of weeks. 
> Actually had hoped it would be on by now, but a bout with the flu and a lot 
> of travel with a new job has interfered with the fun.
> 
> I'm thinking of building both a vertical and a loop antenna just so I can 
> tinker with both styles. We have 20 acres and there are parts that are 
> heavily wooded. Also the portion near the house is not the highest part of 
> the property (but not down in a hole either).
> 
> My shack is in the basement and about 50 feet to the side is a fairly good 
> sized (probably 50 x 50) pen where we keep the dogs when we don't want them 
> running the ranch. It is constructed with t-posts and welded wire.
> 
> There are a few short trees to the back of the pen, but basically I've got 
> tall trees on both sides of the pen with a clear shot across. The trees are 
> pines and they sway heavily in the wind.
> 
> For the vertical, what I'm thinking is running about 30 feet up and then 
> having a T at the top with maybe 20 feet on each side. (Staying within the 15 
> meter total, just throwing out rough numbers. But also using what I guess 
> some consider a liberal interpretation that you measure the radius at the 
> top, not the diameter.)
> 
> One question I have is, does it really matter if the 30 feet are "vertical", 
> or could they slope and still have the T for top loading? I'm thinking I'd be 
> ahead to slope towards the corner of the dog pen so I can use the fence wire 
> as the ground instead of having it all drop down in the middle of the pen.
> 
> Also, is there value in having a loading coil up where the T is formed in 
> addition to the coil used for matching at the bottom? I was going to build 
> the insulator of the T out of something like 4" PVC and wind a foot or so of 
> coil on it in addition to using it for mechanical strength.
> 
> One the loop, there are a pair of trees about 40 feet apart and probably 60 
> feet tall or so on one side of the dog pen. I'd like something easy to 
> install and work on and I've had my share of frustration with slingshots, 
> bows and such. So what I was thinking was getting around 50 feet of PVC pipe 
> to put at each end. Basically I'd hook the side of the loop to the PVC pipe 
> and then strap the pipe to the tree. I was going to try to get the bottom of 
> the loop about 10 feet off the ground.
> 
> If I went that route, would there be any advantage to having what amounted to 
> loading coils in the side of a loop? I could either have a gentle wrap (a 
> turn a foot or so) up the entire length or I could put in more 
> traditional "coils" bottom, middle, and/or top. I was thinking of using 2" 
> PVC (because I've got some handy) with a gentle loop up the entire length. I 
> have no particular reason for going that route, it just somehow strikes me as 
> an interesting thing to try.
> 
> I've also thought that I might use two or three conductors up the PVC in 
> parallel to reduce the resistance in that portion of the loop.
> 
> At this point, I'm just kind of tinkering. I figure I'll try a couple of 
> different things that are fairly "low effort" and then put more effort into 
> the approach that seems to get the best results.
> 
> I have got most of the hardware built. The only thing I need to do is build 
> a "driver" circuit that will take the output from the crystal oscillator out 
> over some coax to the final which will be at the antenna location. Right now 
> I can drive the final with three or four feet of coax, but need something 
> with more umph to drive 30 or 40 feet of coax. Keyer is a PICBeacon with 5 
> wpm, QRSS-10, QRSS-30, or QRSS-60 being selectable.
> 
> Any antenna suggestions would be welcome. And like I said, looking more to 
> tinker instead of for the definitive "this is the best" right now. With all 
> the trees, I'm thinking the loop may be where I end up, but who knows. After 
> all, that's how we justify going for Part 5 licenses from the FCC at a later 
> point, isn't it? :-)
> 
> 73 de
> Jim W4JBM
> 
> http://www.geocities.com/w4jbm
> 
> "With a soldering iron in one hand, a schematic in the
> other, and a puzzled look on his face..."
> 
> Working the world from the New Dog Iron Ranch!
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