[Harc] Antenna Elmer needed

Howard, KB6NN kb6nn at yahoo.com
Sat Apr 25 16:30:17 CDT 2020


Ok now... What I think may have happened is one of these 4:
a. you pressed the tx button while no antenna was connected (connector could have not been properly screwed on, etc.
b. you pressed the tx button while there was a dead short to ground at the antenna connector - a real transmitter killer - easy to test for by unscrewing the coax from the radio and using a standard volt-ohm meter (https://en.wikipedia.org/wiki/Multimeter) to test for short at the radio end of the coax - if found, detach your antenna at the other end, and do the test again - if no short (coax itself and its connectors are not shorted), test the antenna itself - if short exists at antenna itself, check internals of the center insulator or whatever is at the coax attachment point at the antenna end.  If you didn't use a device at the center point, but split the coax, still look for any braid-to-center-conductor short.  One reason to use a center insulator device or balun etc. is that you can easily detach by unscrewing the coax from it
c. this is less likely but... your formula calculates the length of a full wave, which is 2 meters, and 1/2 of that is a half wave (the length of a half wave dipole), and Each Leg should be one half of the half wave (each leg of the half wave dipole should be a quarter wave).  The final size of each leg should be 18-19 inches, not the final total length
d. mismatches: if mismatched, I'm thinking you would have to be key-down for more than a second to fry the finals - txing on high power for more than a few seconds might do it, but if you only tested the radio on UHF, a high SWR might be present if using an antenna made for VHF.

I second Jaye's commendation for your building a homebrew antenna.  Way to go!  HARC had a workshop on building 2-meter antennas out of 14ga house wiring.  I thought I had photos, but have spent a half hour looking for them on my computer to no avail... wait... phone! ...aw, nothing there...

The workshop featured building a 1/4 wave ground plane for 2 meters.  Similar to the image attached, which is from: http://www.hamuniverse.com/2metergp.html

A quarter wave 2-meter ground plane usually works on UHF, whereas a 2-meter half-wave dipole usually does not.  I don't have a reference for that and google searches usually compare 1/4 to 3/4 wave ground planes or 1/4 to 5/8 wave ground planes.

A half-wave dipole is where the two legs *add up to* a half wave.  If each leg were a half wave, you'd have a full wave dipole.  Why use half-wave?  Because of impedance matching.  You can look that up for yourself.

Any piece of wire or metal will try to radiate RF connected to it.  If you want maximum transfer of power, (either to make your radio happy, or just because) you need to consider where the feed point is located on the wire when making an antenna.  I am talking about electrical length, not physical length.  Yes, you can cut a wire to a half wavelength or a full wavelength (electrically), or anything you want, but the impedance will be different, depending on where (electrically) you feed it. That means, for example, if you cut the wire in half and attach each half to your coax, you will get one impedance, compared with cutting it at some off-center point and making your coax connection there, you'll get a different impedance.

RF energy is A/C, not D/C, a principle that I forgot when I was a novice and asked a learned ham who my HF vertical antenna didn't work as I expected.  I said I had it grounded where the coax attached at the edge of the roof, and I drew him a picture of the lamp cord I had running from the ground lug on the coax attachment down to a copper rod I had driven into the soil.  He told me it was probably a good D/C ground, but RF is A/C.  Then he drew over my diagram with a sine wave on end, showing that, at a particular frequency, the zero voltage point will appear several times along a wire (if the wire is long enough).  So for you, try drawing a horizontal line on paper, then overlay it with a sine wave.  We feed half-wave dipoles at the center.  Think about it.  Half-wave.  Draw a half wave not a full sine wave.

Recommended reading: The ARRL Antenna Book or the antenna section of the ARRL's Radio Amateur's Handbook (any edition).  Mail order available, and the new handbook is usually very expensive when it first comes out http://www.arrl.org/shop/ARRL-Handbook-2020-Softcover/.  But we are sheltering, so also consider: https://www.universal-radio.com/used/usedb.html or https://www.ebay.com/b/arrl-handbook/bn_7024902254 - make sure it is the ARRL handbook, and not Ham Handbook or Ham Radio Handbook, although I haven't read any of those, so maybe... But the real deal is the ARRL Handbook.  If you are not a member of the ARRL, you should consider joining: http://www.arrl.org/membership

Ok, I'm being called from the other end of the house.  I took way too long to compose this email, but I was trying to be as thoughtful as possible, and hope you find it helpful.

73 and Shelter,
Howard
--
KB6NN


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