[TowerTalk] Fw: quad tuning, etc. et al

Sun Jun 25 00:13:53 EDT 2006

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>From: ac5e at comcast.net
>Sent: Jun 24, 2006 4:38 PM
>To: HAMQUADS at yahoogroups.com
>Subject: Re: [HAMQUADS]  quad tuning, etc. et al
>
>Trying to tune any antenna with elements tied together has always been an exercise in futility for me. Trim one element - and the next is now too long. Or short. Or no longer seems to be connected. Or something! You will be much happier with separate loops, since they will behave as you expect, not as you wish. 
>
>I second the suggestion to remove the balun. Don't throw it away, the core may come in handy some day. 
>
>But do use a remote control switchbox that leaves the unused driven loops OPEN  to change loops/bands.  That is, if the 20 meter driven loop is in use all the other loops should be open. Neither antennas nor transformers work well with a shorted turn in close proximity to the primary, and RF power is too expensive to heat shorted loops with. 
>
>Even with all unused DE's open I have had much better luck running my various and assorted quads with only 20/15/ and 10 Meter loops.  The pattern is much cleaner, and the FB is better. Sorry, I could not strip the 17 and 17 Meter loops off fast enough to make a direct comparison in gain. 
>
>You can use either Bill Orr's loop lengths, the ARRL Antenna handbook sizes, or calculate them yourself with a five buck calculator. I do suggest finding a 100 foot steel tape with incremental inch markers to the 1200 inch end, and multiplying the 1005 in 1005/f by 12 (giving you 12060/f) to give you loop lengths directly in inches. One gets you 71.02474 feet, the other 852.3 inches for a quad's DE at 14.150.  Not having to convert from one to the other saves time and uncertainty. 
>
>High quality 72/75 ohm RG11 is recommended for quarter wave sections. This will have a velocity factor (propagation velocity as a percentage of the speed of light) CLOSE to .66 - and the foam core version will have a VF close CLOSE to 0.82. Lower quality stuff is likely to be further from nominal - but if you don't have an Autek, MFJ 259 or some other antenna analyzer you can make sure you have the good stuff and use antenna manual lengths.  
>
>If you do have an analyzer, cut the coax "long," connect it to your analyzer, and trim until you get the LOWEST impedance possible on the analyzer. Verify the length by shorting the end, and the impedance should become very high. 
>
>And, if you don't want to run the full legal limit, smaller size "75" ohm coax such as RG59 will also do. But the stuff I have measured is variable enough to make me throw the cookbook away. 
>
>The third element is well worth while - and the longer the boom and wider the spacing between spreaders the happier you will be. Quads are funny, you don't gain much with four elements - but if you can manage 5 with at least 10 feet of spacing (40 foot boom) you should be a very happy camper. 
>
>If you are interested, besides Bill Orr's and the various handbook articles, Bob Haviland's "The Quad Antenna" is an excellent resource as are the Cebik books available from AntennX. 
>
>73  Pete Allen  AC5E
>
>Amateur Radio, emergency communications when all else fails
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