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Author Topic: Sometimes a blob is better.  (Read 190 times)

Offline Josh

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Sometimes a blob is better.
« on: May 04, 2022, 2044 UTC »
A HAM friend and I were discussing antennae of various bands and configurations one day. He mentioned once having a 600ft dipole up for a while, the center insulator up about 75ft on the tower. The ends were secured to some fencing used to pen the cattle. I was awestruck by this wonderous dipole revelation and asked how it performed, expecting legendary dx from such a monstrosity.


He ran it on all the lower bands and it worked ok but not much better than the dipoles had had curt for each band, and expected it to be the cats meow on 10m as that was the band he had no antenna cut for. He heard no one on 10m and apparently no one heard him despite hours of calling. He wondered wtf was going on as he had multiple wavelengths of dipole out there to get massive gain on 10m but nope, nada, zilch.
So he finally ran a antennae sim and saw wtf was going on.
No one was going to hear him (nor he them) unless the receiving station was directly in line with the legs of that fabulous dipole. if he had simply moved the legs so it beamed, for example, into JA land he might have the killer signal in Tokyo. Did I mention when the wind blew, or even worse a storm was imminent, you could arc weld with the socket at the end of that titanic dipole's coax? When he saw that he decided it was a bit too dangerous of an antenna to let live.

Then as he was still speaking I recalled the simple 2wl wire I ran on 11m, up about a wavelength. A guy across town had me s9 plus on that antennae while a guy two blocks from him never heard me and wondered who he was talking to - yet I could hear them both fb, several others in town couldn't hear me either. All could hear me when I went vertical.
I modeled a 2wl wire on 10m and noted the many and deep nulls, explaining the reception oddity noted, and here I thought my breath just smelled bad.

So.... sometimes an undefined blob of rf (here meaning a halfwave dipole or wire) is better than rf pattern with steep jaggy edges, depending on what one wants to do with it of course.

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Offline ThaDood

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Re: Sometimes a blob is better.
« Reply #1 on: May 07, 2022, 1822 UTC »
A blob-like radiation pattern is how I talk out of this river valley bottom. Certainly, not an ideal place for radio hobbies, but it does pose a challenge, at the very least. Especially, in the VHF / UHF bands.
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Offline Ray Lalleu

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Re: Sometimes a blob is better.
« Reply #2 on: May 12, 2022, 2122 UTC »
An antenna arm that is half WL (wavelength) long has a pattern with only 2 lobes.

A 1 WL long arm has a pattern with 4 lobes.
A 1 and half WL long arm has a pattern with 6 lobes
A 2 WL long arm has a pattern with 8 lobes
and so on, add 2 lobes for each half wave added to the arm length.

As the wire becomes longer and longer, the stronger lobe is closer and closer to the wire.
Knowing this pattern is the key to design long V antennas and rhombic antennas, so that the main lobes of all the wires add together in the axis, and the other lobes more or less destroy one each other. The apex angle of the V or the rhombic is chosen to get the best combination of the main lobes (to some elevation angle).

If you use only a straight long wire, you will get a pattern like a daisy flower !

So, usually there is no reason to use center fed antennas longer than 1 WL (apart convenience for multiband use, or well chosen different directions). For better results to long distances, the right way is to place the antenna as high as possible, giving better efficiency, and better low angle gain.

Beware! For simple HF antennas, gain is a misleading idea. More important is the efficiency (also in receiving, as a receiving antenna is also radiating), and about that, many simple antennas are far in the negative range of effective gain. More, for receiving, that effective gain is only next to the noise pickup, the right idea being to get the best S/N ratio rather than the highest S level.
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