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need help designing yagi

need help designing yagi

need help designing yagi

Ok, so I'm building a completely adjustable, lightweight, field portable yagi antenna. As of yet, I am not utilizing any baluns or matching devices in order to reduce weight and decrease the complexity of tuning. I am utilizing a straight dipole for the driven element (it is not folded) and feeding it directly. I am modeling my yagi to be adjustable in both element length and element spacing along a long square boom as to adjust for any desired band/frequency. The entire antenna is designed to be broken down into 3 foot sections for ease of portability. I really like the mechanical design, but I'm not too sure how electrically efficient it will be.

I want this antenna to be as versatile and fool proof as possibly. I am using yagi design software to get a base configuration to start with for any particular frequency I desire. I do not expect this to be a high performance antenna like some expect their yagi's to be.
I am modeling mine similar in design to a military version like the one in the following ebay link: http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&_trksid=p4340.l2557&hash=item20c85fc63a&item=140800673338&nma=true&pt=LH_DefaultDomain_0&rt=nc&si=hShFVRiFN8XyjqEwoNkuHTmrAkI%253D&orig_cvip=true&rt=nc

My questions are as follows:
• What is the best way to fine tune the yagi?
o I read that only the dipole should be adjusted to lowest SWR and to leave the reflector/director element length and spacing alone.
o I also read that to tune the beam on the ground you should point it skywards with the reflector on the ground.
• How do I know when the yagi is actually in tune?
• Is SWR alone the only/best indicator that the antenna is in tune?
• The best SWR I have gotten yet has been 1.5, is this a decent SWR for a non-perfect yagi?
• If I end up using a balun or choke, do I tune the antenna with the choke in place or disconnected?
• By not using a balun, will the common mode currents pose a threat to the radio even if the SWR is low?
When I tune the dipole alone (off the boom) and away from the other elements, I find that the point of resonance is pretty far out from the theoretical/ideal calculation. For instance, if tuning it to 440Mhz, I find I need to adjust the element beyond the theoretical length by approx 20Mhz.
• Is +/-20Mhz at 440 ok, or is something way out of whack?

-As of now, the boom is made of 4 three foot sections of 1" square tubing totaling 12 feet long when assembled.
-I have a total of 5 elements (including the driven) that are insulated from the boom.
-The elements consist of a 1/2" OD outer aluminum tube with a 3/8" OD inner telescopic tube


RE: need help designing yagi

You should review this thread.

The two main points are that, 1) a Yagi antenna has a large number of variables making it tricky to adjust them all to their optimum values in the field, and 2) final tweaking of the match ("SWR") can also be accomplished at the transition from the feed line to the driven element (there are a large number of matching systems available, direct connection is just the very simplest option).

Quote (jared1313)

...any desired band/frequency...

"Any" is a bit ambitious. The ratio from maximum to minimum frequency for such an antenna is not likely to reach double-digits. Thankfully, it's not likely that the system requirements would need such a wide range. One is normally working on an HF project *or* a V/UHF project. It's actually extremely rare that a single antenna system would attempt to cover (for example) both HF and V/UHF.

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