[Harc] Antenna Elmer needed

Sat Apr 25 18:07:00 CDT 2020

https://youtu.be/RcO5WAiksNI

I made this one. It's working well.

73,
Jason

On Sat, Apr 25, 2020, 2:30 PM Howard, KB6NN via Harc <harc at humboldt-arc.org>
wrote:

> 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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