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Author Topic: Fractal Antenna for 28 Mhz - help needed  (Read 10456 times)
AG1LE
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« on: December 24, 2011, 02:44:05 PM »

After reading eHam and QRZ forums and searching Internet for some practical examples of fractal antennas I decided to create a quick fractal quad prototype for 28 MHz  to learn more how this type of antennas work in real life.

I documented this  antenna prototype project  and measurements I have done  in the blog:

http://ag1le.blogspot.com/2011/12/antenna-experiments-fractal-quad-for-28.html


Since I am not an antenna expert and I haven't really used modeling software before this project I would like to get some feedback & comments from real experts / elmers in this forum.

My questions:

1)  Has anybody built a 2 element quad for 28 MHz  and could you share the design dimensions please?  I would like to compare the performance of this prototype to a full size quad.

2)  I have a NEC2 model and some measurement results posted in the blog  - what instruments / methods should I use to verify the antenna performance against the model?   I don't have  instruments other than AIMuhf antenna analyzer, CN-801 SWR meter and I have very limited space (165ft x 50 ft size lot) to do any measurements.

3)  I tried using WSPR to get comparative SNR reports from many stations from different locations - it seems that band conditions at 28 MHz create too much variation to have any meaningful comparison data.  Is there a better way to compare two antennas?

Thanks in advance for you help.

73  de  Mauri,  AG1LE

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WB6BYU
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« Reply #1 on: December 24, 2011, 03:03:57 PM »

The most comprehensive quad studies I know of were undertaken by the late W4RNL and
described on his web site here:

http://www.cebik.com/content/a10/quad/quadlist.html

He developed "optimized" quad designs with 2, 3 and 4 elements (aiming for a balance of
gain, bandwidth, and F/B ratio) and then created some equations that will generate the
dimensions for any given frequency and wire diameter.  The latter is very important -
if you see a quad calculator that doesn't include it, don't trust the results.

You can download the spreadsheet from W4RNL's site that includes the formulas, or there
is a quad calculator online that implements it (though I've found a couple bugs in it.)  I've
built these quads for 121.5 MHz through 732 MHz with good results and no tuning required.

An example for 10m would be 111" on a side (55.5" half width, which is more convenient
for modeling) for the reflector and 105" on a side (52.5" half width) for the driven element,
with 65" spacing using #14 wire.  This models to 7.1dBi gain and 36dB F/B at 28.5 MHz.
Feedpoint impedance is 128 ohms at resonance, and a quarter wave of 75 ohm coax matches
it to better than 1.6 : 1 across the band from 28 to 29 MHz.


You'll find lots of other interesting stuff on W4RNL's site - yes, you have to register, but it is
free.  For example, try these:

http://www.cebik.com/content/a10/ant21.html
http://www.cebik.com/content/a10/quad/40q.html


Unfortunately our company just arrived, so I can't get back to the rest of your questions, but will
check out the blog entry as circumstances permit.
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N3OX
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« Reply #2 on: December 24, 2011, 03:40:19 PM »

2)  I have a NEC2 model and some measurement results posted in the blog  - what instruments / methods should I use to verify the antenna performance against the model?   I don't have  instruments other than AIMuhf antenna analyzer, CN-8
http://www.g4hfq.co.uk/plphelp/plphelp.htmhttp://www.g4hfq.co.uk/plphelp/plphelp.htm01 SWR meter and I have very limited space (165ft x 50 ft size lot) to do any measurements.

Mauri,

If you have some kind of small power signal source (your AIM could suffice) you can do comparative measurements against another antenna using your rig and a voltmeter.   It helps if you can turn the AGC off on your rig, but if you can't you can just use a signal that's weak enough to be below the onset of AGC action.   See http://n3ox.net/projects/n3oxflex/ for a comparison measurement I did between two 40m verticals on a small lot.  

If you swap the voltmeter in the test setup on that page for a sound card and G4HFQ's polar plot (http://www.g4hfq.co.uk/plphelp/plphelp.htm) you can take patterns easily to test the F/B.

Here's a slightly weird but illustrative pattern I took of my 15m Moxon using Polar Plot.  

http://n3ox.net/files/15mox_messed.jpg

The weird bumps in the forward lobe aren't something I tracked down yet.  I thought they were something to do with the fact that I wasn't deep in the far field, but that wasn't it (I generated nearfield patterns with EZNEC and they were pretty much the same as the farfield pattern at that distance).  It's probably just the influence of other antennas in my yard.

Measuring antennas and getting meaningful answers is more about measurement protocol and being willing to put in the work than it is about fancy equipment.  Simple equipment can give very accurate results, and checking repeatability by doing the same measurement on the same antenna several times can give you a good idea of the size of the unavoidable errors in your measurement setup.

Putting the signal source and its antenna in different places around your yard can give you some idea of the influence of nearby objects on your imperfect test range. If you're comparing two horizontally polarized beams that are reasonably in the clear, the influence of random objects on the test results should be similar, but it's good to do several trials where you change NOTHING explicitly (to check for randomly changing influences) and then several trials where you change something that shouldn't matter, like the location of the signal source at a fixed distance from the antennas under test, to check for excessive sensitivity to small positioning errors, etc.

===

On air tests with WSPR could be a good supplement but I think you probably want to take many hundreds or thousands of results and see if the trend emerges.  Propagation has such enormous variations compared to the difference between a small beam and a comparison antenna.  Here's a "pattern" I took in Polar Plot by pointing my 21MHz beam at 20MHz WWV and just leaving it pointed there, stationary for one minute:

http://n3ox.net/files/20MHz_WWV_fading.jpg

Here's a probability distribution of WWV signals on 10MHz over something like 10-15 minutes:

http://n3ox.net/files/wwv_fading.png

These things are the TYPICAL situation for just steady conditions... doesn't require the sun to be setting or any transient.  They're just typical skywave variations that need to be averaged over to get a reasonable result.

I think things like WSPR have a lot of promise, but they might work better as part of an automated skywave testing toolkit where you left your stuff running for a few days and switched between antennas every couple minutes to take thousands of automatic measurements.

Line of sight results with a controlled signal source will tell you a lot and any single antenna A vs antenna B measurements will have random error of the order of 1dB or 0.5dB or 0.2dB, not 10dB or 20dB.

« Last Edit: December 24, 2011, 04:03:09 PM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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« Reply #3 on: December 24, 2011, 05:08:54 PM »

The insulation material and thickness affects the resonant frequency, as does the wire
diameter.  That might be why your measured frequency is lower than the calculated one.
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AG1LE
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« Reply #4 on: December 24, 2011, 05:30:01 PM »

...
If you have some kind of small power signal source (your AIM could suffice) you can do comparative measurements against another antenna using your rig and a voltmeter.   It helps if you can turn the AGC off on your rig, but if you can't you can just use a signal that's weak enough to be below the onset of AGC action.   See http://n3ox.net/projects/n3oxflex/ for a comparison measurement I did between two 40m verticals on a small lot.  

If you swap the voltmeter in the test setup on that page for a sound card and G4HFQ's polar plot (http://www.g4hfq.co.uk/plphelp/plphelp.htm) you can take patterns easily to test the F/B.

Dan

Thanks a million for this above...I downloaded the PolarPlot, configured my Flexradio virtual audio cables to suit PolarPlot audio input  and tested the setup using AIM "constant Freq" function per your recommendation.  It worked exactly as you indicated above, different AIM output levels show nicely on the plot....looks like I have now the tools & software I needed and thanks also for the links you provided. This is very useful information and thank you for great examples!

73  de Mauri, AG1LE
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AG1LE
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« Reply #5 on: December 24, 2011, 07:40:16 PM »

...
An example for 10m would be 111" on a side (55.5" half width, which is more convenient
for modeling) for the reflector and 105" on a side (52.5" half width) for the driven element,
with 65" spacing using #14 wire.  This models to 7.1dBi gain and 36dB F/B at 28.5 MHz.
Feedpoint impedance is 128 ohms at resonance, and a quarter wave of 75 ohm coax matches
it to better than 1.6 : 1 across the band from 28 to 29 MHz.
..
Thanks Dale
I registered to the site and created a NEC2 model of this 10 m quad http://www.cebik.com/content/a10/ant21.html using CocoaNEC software. For some reason the resonance frequency with these dimensions is off, at 27.9 MHz. Also, the radiation pattern appears similar, but at 29.7 - 30 MHz  instead of 28.25 - 28.75 Mhz like in the article.

I double checked dimensions, wire diameter (#14), height 35', spacing 60",   etc. - according to article "As shown in Figure 1, a good 10- meter quad can be built from two wire loops. If the wire you use is #14 bare copper (stranded or solid), the driven element loop is 105.3" per side or 421.2" overall. The reflector loop is placed 60" behind the driven element and is 110.3" per side or 441.2" overall."


There must be something else that I am missing here.  What could cause this difference?

73  de Mauri, AG1LE

   
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KE4YOG
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« Reply #6 on: December 24, 2011, 09:58:30 PM »

This is another case of unexpected help. I am getting started on a new project since Christmas is almost over. Thanks for the information. Another case of what ham radio should be like.
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WB6BYU
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« Reply #7 on: December 25, 2011, 12:01:58 AM »

Quote from: AG1LE

 For some reason the resonance frequency with these dimensions is off, at 27.9 MHz. Also, the radiation pattern appears similar, but at 29.7 - 30 MHz  instead of 28.25 - 28.75 Mhz like in the article.





Are you feeding it in the correct element?
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AG1LE
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« Reply #8 on: December 25, 2011, 05:03:55 AM »

Are you feeding it in the correct element?

Thanks again, I was feeding it in the reflector element. I changed that to the driver (shorter element) and now I can replicate the W4RNL results. 

Thanks also for pointing out the importance of wire diameter on quads. 

For the fractal quad prototype I used 2+1 wire  #14 AWG outdoor electrical wire from Home Depot. It has insulation on and there is a center (ground) wire between the two power wires. All the wires are connected together at the feed point and soldered to SO-239 connector. 

How would I model this wire?    Do I need to create separate wires in the model or can I just use   3 * 0.032" as radius for the wire? 

The fractal model has 100 wires per element as the shape is complicated.
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WB6BYU
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« Reply #9 on: December 25, 2011, 11:08:32 AM »

Modeling that gets complicated - the insulation probably makes more difference than the wire
diameter, and we don't know much about that.  When in doubt, probably the best approach is
to model the antenna and simply scale it to match the resonant frequency of the prototype,
and assume the same frequency ratio (which won't quite be true, but may be close enough.)

The NEC4 software can handle insulated wires.

One of W4RNL's articles discusses the effect of using multiple wires in place of a single one,
and I think he calculated an effective wire diameter, which wasn't a nice linear function.
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N3OX
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« Reply #10 on: December 26, 2011, 06:38:45 AM »

The NEC4 software can handle insulated wires.

NEC-2 can too.  I don't know if CocoaNEC includes that ability.  There are lots of things NEC-2 can do that various implementations can't.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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