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I've tried building three shortened 40M antennas, and the one that works the best (by quite a wide margin) is something along what Alpha Delta does - using coil loading that is out quite a ways away from center.

Performance is just on the air - and is simply the antenna that sounds "hottest" and also where I can make one call and work just about anything.

My guess is that linear loading is OK - but not where the entire antenna is essentially one large inductor. The Shorty40 with center loading is very hard to get a match and extremely narrow banded, but having a set of coils out towards the end of the antenna - where you haven't shortened too much (i.e 70% of a full sized dipole) seems to have the best compromise between bandwidth and efficiency.

If you've had experience with the Cobra, Alpha Delta shortened dipoles or the Shorty40 - please let me know.

Thanks,

Rich
KY6R

[AA4PB]

Over the years I've tried a number of shortened 40M and 75M dipoles. If you don't shorten them more than 50% of a full sized dipole then the efficiency doesn't suffer too much. I don't notice the difference between a full sized and half sized dipole, other than the more limited bandwidth.

The affect of the position of the loading coils is the same as in a mobile whip. Center loading provides better current distribution and better efficiency, all else being equal. The size of the wire used in the loading coils has a major impact on efficiency and bandwidth. One problem with many of the continuous loaded antennas is that they generally use very small guage wire.

[WA7NCL]

The Cobra relies on using an antenna tuner.  It has supposed magical properties due to "extra resonances".  

You could use an antenna tuner on your shortened antenna.  If you use low loss feed line it will work fine.  You could improve your shortened antenna if you can add capacity "hats" on the ends.  You then need less loading inductor and the feed impedance will be higher.  You could add drooping wires to add end capacitance or add wire "Tees" to the ends to increase capacitance.

I believe most books and practical studies have shown so called linear loading is not any better than properly designed lumped circuit loading.

I have tried capacity hats on various shortened vertical antennas I have built.  It always has made them work a lot better.  The same things will work with dipole type antennas, you just need two of them .

[NS5M]

I've had an Alpha-Delta DX4020 in the attic for almost 4 years, and a Hustler 6BTV in the yard for about 2 years. The DX4020 (the top "rung" of the DX-EE) has served me quite well on its designed bands (and even a few times on 30m).

Both antennas have met or exceeded my (I think reasonable) expectations, and show the characteristics you'd expect. The vertical is "noisier" than the short dipole and I've worked lots more DX with it (6BTV). The DX4020 has bagged a smaller share of DX, and most of my WAS entries. Remember that the DX4020 is in the attic, with all of the electronic distractions normally found there - ductwork, cable, electrical power, etc - but has still done quite well.

The antenna is coax-fed (bury-flex) and driven with 100W, oriented N-S at about 16-18 feet above ground level.

Don't know what sort of information you're seeking, but if it's anecdotal ya want, I'm yer guy.

73,
Jim N5VT

I had two of the Alpha Delta 40M coils - so built a version of their 40/80M dipole (82' long), but fed it with 450 ohm ladder line to a heavy duty 4:1 balun and coax into the shack. The antenna is up just about 60 feet in the air, and it seems that once I got it above its old height - which was 37' - then it really started to outperform even the best vertical I had tried previously. No wonder - its up almost 1/2 wl on 40 - a height I could never reach at my old QTH.

I have had big success in the past with shortened, top loaded and hatted verticals (DXE 24' 40M vert with 60 radials and big cap hat, Force-12 Sigma vertical dipoles and the Cushcraft MA-8040V.

The 82' loaded doublet that I built seems to easily beat the homebrewed Cobra (45' with 3 conductor rotor wire) and center loaded shortened 40M "shorty 40" (37' long).

The only other 40M antenna that kicked butt was a pair of DXE verts that I phased end fire and broad side using the DXE 8 position switch.

But now I have tall trees and live down in a crook between steep hills with a creek and redwoods all around.

It seems that getting a wire up 60' is much better than verticals at this location. I used to live on the top of a hill with a big flat field - and so verticals were better there.

I think I like the tall trees I have now - much more - for antenna fun.

[K3JVB]

alpha delta

dxcc, 82 feet 10-80 meters. Good on all bands. A little narrow on 75-80. Need a tuner to have usefull band width there. But a decent performer. Will tune 17 and 30 meters. But not real efficient on those bands.
JohnB
k3jvb