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Antenna Modelling

One of the problems with theoretical antenna modelling is that it is so much easier to design something on the screen than to get it to work in the field.

Inspired by EZNEC, I set about designing a novel loop aerial for 40 metres. The concept was to have something directional in the same space as a low quad-loop. The results are below. It looks great but... when it came to building it I failed to get it to work. This sort of critically coupled aerial is - well, critical! I guess that my test equipment was just too inaccurate for the job. I tried for days.

wpe6.jpg (36630 bytes)

wpeA.jpg (34206 bytes)

wpe9.jpg (34554 bytes)

The design is compact, low, has a clean pattern and is easily switchable with relays and stubs at low level. A neat feature is that effectively each side is an inverted L with a single loaded radial. The loading coil is connected in series with the "radial" in the fed element and between the inverted L and the "radial" in the parasitic section.

Here is an EZNEC 2.0 file for it. I have several versions, all rather similar. The idea is that only one element is fed and a coil is inserted across the feedpoint of the other loop. I devised a  switching method with halfwave stubs and DPDT relays.

Can you get it to work (or tell me how to)?

Post Script - 23 September 2000.

I have just bought an MFJ-259B antenna analyser. This should give me some more detailed insight into my aerials. So, help me out guys and girls. Should I try this aerial again or should I give up? Let me know below.


Should I try again?

Yes

No



View Results

Comments Received

From: cebik <cebik@utkux.utcc.utk.edu>
To: Richard Newstead <g3cwi@qsl.net>
Date: 11 October 2000 21:46
Subject: RE: Antenna Problems!
>Richard,
>
>I looked at your design, but have not yet looked inside the EZNEC file.  There 
>are a few items that can prevent the antenna from working, some of which can be 
>temporaily eliminated to establish the baseline--and then readded if 
>necessary.
>
>First, omit the stubs and any other apertenances no critical to basic antenna 
>functioning.
>
>Second, check the loading coil mountings.  I got the impression that they were 
>mounted across the feedpoint on the parasitic element.  Since feedpoints tend 
>to have a bit of capacity, I hope the loading inductor is not being resonated.
> In any event, adjustability is critical and if the load coil is a large 
>value, whatch out for self-resonance due to inter-turn capacitance.
>
>On the fed element, the coil should be in series with the feed on the radial 
>side, as you say.  However, the lower wire is not just a tuned radial.  It is 
>also a horizontal radiator, as evidenced by the current level on it and by the 
>horizontal component of the azimuth pattern.
>
>Attaining in the actual antenna the depth of null in the model is likely 
>impractical, since a nearby breathing animal may be sufficient to push the 
>actual antenna off deep null.  When making models, be sure to check the 
>bandwidth of the desired pattern--or what you establish as an acceptable 
>pattern.  Some designs are so narrow-banded that performance is nearly 
>impossible for more than a few seconds (the space between that animal inhaling 
>and exhaling.  One key to successful compact antennas is a reasonable 
>bandwidth for the pattern and the feedpoint impedance so that adjustment is 
>not super-critical.  In fact, the further out of reach the antenna is, the 
>broader the desirable bandwidth so that one need not interrupt operation to 
>readjust the array.
>
>Also, in your models, be sure to convert to RLC loads, with sufficient R to 
>estimate coil Q to check the bandwidth.  The R-X type loads give an 
>unnaturally larger bandwidth, since they do not account for changing reactance 
>of a coil with changes in frequency.
>
>Hope these notes are useful in getting the project on the road to being 
>successful.
>
>-73-
>
>LB, W4RNL
>
>L. B. Cebik, W4RNL      /\   *    \   \  \           tel: 865-938-6335
>1434 High Mesa Drive   /  \/\ /\   \---\--\      e-mail: cebik@utk.edu
>Knoxville, TN 37938   /   /  \  \   \ /\\  \    e-mail: w4rnl@arrl.org
>U.S.A.               /   /    \  \    ||     URL: http://www.cebik.com

 

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