Multiple Dipoles Single Ladder Line Feed Questions

[K1JJ]

Hola Steve,

Looks like an interesting design. Are those shorted stubs to make the 160M legs electrically longer to make a 1/2 wave?   And, the ant is fed in the center, right?

I was thinking of another idea that would play well with your design.  For example, on the east coast, we might like to favor west to cover the USA with a broad 2el Yagi pattern on 160-10M.  Put a set of six reflectors behind your driven elements using the same spreader idea. You might group them as 160-40M as one triple set 45' away. The next triple set for 20-10M could be mounted 12' away.  There would be some interaction between the reflectors, but using modeling software the legs could be tweaked to work as reflectors well, something like an "interlaced" Yagi design.  Lower the reflectors to make it a regular dipole pattern again. 


Tom: Now you're talking. That antenna modeling software will open your eyes. Use a 1/2 wave dipole as a starting point for any antenna and as a reference.  Build on it by adding a reflector and director and then stacking them. Try feeding them with different phases and heights to see what happens.

It's surprising how well a simple 1/2 wave dipole works. It's far better to have a broad figure eight pattern than to gain 2 db but lose 10-15db in other directions by lengthening it.  As you found, when the antenna becomes longer than a wavelength or so, the pattern starts to get crazy. It "works," but so does 8 men paddling in different directions in a boat. A lot of wire in the air is meaningless unless it is controlled and phased correctly.

Have fun with the new software.

T

[K3ZS]

Tom:   I don't have space to put up a 10M reflector using wires.     I actually have too many trees, the 135 ft span is the only location on my property.    Someday I may just put up a 10M beam on the garage.

I had another question about the EZNEC program.    The free version allows only 22 segments, so I get a lot of error messages about segments being too long, but the patterns still look like they should.     What kind of errors are produced when segments are too long?

[KA1ZGC]

Yep, I'm glad I finally got my hands on a modelling package. There are countless different packages out there, and they're all built around different versions of the NEC core; so it's just a question of the GUI, and what other packages it will talk to. I wound up with 4NEC2X because it will interface directly with the ItsHF propagation modeller, which lets you actually see your likely signal strength at various freqs at various points on the globe.

Having a 3D model of your radiation pattern is a huge bonus, too.

Anyway, back to the ant: I've been modelling the 200 footer because that's what I've got hanging (caw mawn). I can't reasonably put a reflector or director in the air, so I'm trying to see what I can do to fill in the nulls from 20 meters up by adding a shorter inverted vee at the feedpoint.

14.9 meters each leg of the vee (the max length I can fit between the feedpoint and ground at a 45 deg. angle) basically gives me a 60 meter halfwave, which fills in the nulls on 20 very nicely, but has less effect higher up.

FWIW, I narrowed down the point at which the pattern starts getting "noisy" to right around 16 MHz with the current ant. Leaving the WARC bands out of the equation, it needs the most help on 15 and 10, so that's where I'm going to focus modelling the inverted vee part of the ant, then see what damage that does to the patterns on 160 through 20. I suspect that if I start shortening the vee to something more of a 30 meter (or so) halfwave, I might be able to smooth out the pattern above 16 MHz.

The last phase will be sweeping the model to see what the R, X, and Z curves look like to determine how best to feed the thing on different bands, whether I'll need to add any lumped reactances, series vs. parallel feed, and so on.

This is mostly a "because it's there" exercise; I could probably just as easily barf up a dedicated tribander for the higher bands, but if there's a way to shoehorn those bands into my existing ant, I might at least come up with something that does half-decent across most of the HF spectrum I can transmit on.

Steve: That ant looks really interesting. If I wasn't already well-entrenched in the link-coupled balanced-feed domain, I'd be very tempted to give that a shot. Perhaps the next station I build will employ something along those lines.

Glad you guys got talking about this, it gave me the impetus to finally get serious about antenna design, rather than just barfing something up and hoping for the best (though it's amazing how well that can work sometimes).

--Thom
Killer Album One Zappa's Greatest Compositions

[Steve - WB3HUZ]

Looks like an interesting design. Are those shorted stubs to make the 160M legs electrically longer to make a 1/2 wave?   And, the ant is fed in the center, right?

Yes, the stubs hanging down are to make up for the lack of length on 160 meters. The rest of the dipoles are standard half-wave types for each band. The system is fed in the center with coax.

[WA1GFZ]

I've seen a flat top above an inverted vee. one side of the feedline fed the flat top and the other side fed the center of the inverted Vee. Look at as a slice out of a discone. Never simulated it or tried it though.