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View Full Version : 40 meter Inverted V and the 20 meter band.



N4ENT
10-15-2008, 09:29 PM
Ok, I have been reading up on my Inverted V.

But ....I am confused (easily done these days.... :))

I have an half wave 40 meter inverted V fed with a whopping 40 feet of coax fed. (lucky me having everything close) It is 30 feet up with a 1:1 Balun on it. Balanced Open line is not really a option so the question is...

Will this work well on 20 meters? I know 15 is no problem but I am confused on the workability of the 20 with this configuration. Will I lose too much due to harmonics?
I do have a 930s with an internal antenna tuner. It will match both the 40 and the 20 but at what loss?

Thanks and 73!!!

Ed

WB2WIK
10-15-2008, 09:46 PM
At a lot of loss.

A 40m center fed dipole or inverted vee, fed with coax, is an awful antenna on 20 meters. Its feedpoint impedance should be several thousand Ohms, and that mismatch is really there whether your tuner allows your transmitter to load up or not.

The coax loss, even in 40 feet, can be quite high. If you run any real power you might flash the coax at the feedpoint, since at a kilowatt there can easily be >2000 volts across the feedpoint. (Even at 100W, there would be about 600V across the feedpoint, and this could "flash" some foam cables like RG8X.)

I'd either switch to twin lead or make a parallel dipole for 20m, fed at the same insulator and by the same coax, but tuned to 20 meters. That's a pretty easy thing to do: Takes 33' of wire and maybe 8 PVC insulators.

WB2WIK/6

G3TXQ
10-15-2008, 09:52 PM
The coax loss, even in 40 feet, can be quite high.

To put some numbers on it - quick calculations suggest the coax loss alone would be around 7dB for RG213 or 10dB for RG58.

Steve

K1VSK
10-15-2008, 09:57 PM
in addition to the above comments, the radiation pattern would be so screwed up that even if your were to somehow feed it, you'd be pretty unhappy with the result. No such thing as a free lunch.

N4ENT
10-15-2008, 10:05 PM
So running 80 meters in the 40 V is ok? (100 watts max)

Thanks

Ed

AG3Y
10-15-2008, 10:14 PM
You can, however convert that antenna into at least a three band setup by simply adding a 20 meter doublet at the same feedpoint as the 40 meter antenna! The ends of the shorter antenna should be spaced away from the 40 meter antenna by about a foot or so. The spacing isn't super critical, but you do want to have some so the antennas don't interact too much !

This will give you a low feedpoint impedance on 40, 20, AND 15 meters! There will be some interaction, but nothing you shouldn't be able to tune out with a fair antenna tuner. The impedance will possibly not be perfect, but it will be a far shot better than you will get by trying to feed the 40 alone !

Good luck, and let us know how it turns out! 73, Jim

K0RGR
10-15-2008, 10:37 PM
Basically, a coax-fed dipole will work on the band it is cut for and the odd harmonics of that frequency - i.e. 7 and 21 Mhz. It will not work well on the even harmonics, like 14 and 28 Mhz..

Now, a coax-fed full-wave loop will work on both even and odd harmonics, so a 40 meter loop will work on 20, 15, and 10, as well (it will also work on 6 to some extent). It won't work well on the WARC bands or on 80 meters. If you feed the loop with balanced line, it will work on all frequencies higher than the design frequency, and somewhat below it - a 5 Mhz. loop will provide coverage of 80 meters and up.

WB2WIK
10-15-2008, 11:44 PM
So running 80 meters in the 40 V is ok? (100 watts max)

Thanks

Ed

::Where did you get that idea?

It might load up if you have a good tuner, but it's a very lossy system. Try it on 80 and see if you can make a cross-country contact. My guess is "no."

But try a 1/2-wave 80 meter dipole, and get on the band any time after dusk and try it. My guess will change to, "Sure, it's easy."

WB2WIK/6

N4ENT
10-16-2008, 12:16 AM
Thank you all for the info. I will keep you posted Jim.

I had seen the doublet, but was unsure of its effectiveness.


More later!

Ed

N7SGM
10-16-2008, 01:11 PM
Hi Ed,

Steve, WB2WIK, hit the nail on the head. Steve makes the point of simply adding approximately 33 feet of wire for the 20 meter band. This will make you a fan dipole antenna that will work just fine on 20 meters. I know this because I homebrewed this exact antenna along with a coax wound choke at the feedpoint. Actual wire length for 14.250MHz would be 32.8 feet by formula. 468 divided by 14.250 = 32.842 feet or 16.42 feet per side. If you use a tuner, the antenna will work for the entire 20 meter band. 20 and 40 meters worked quite well for me. Don't forget to run the 40 meter wire above the 20 meter wire with approximately 8-10" of separation between the wires to eliminate interference. In fact, the ATU built into my radio also gave me 15, 12 and 10 meters. For some good plans, checkout the hamuniverse.com Antenna Design page for the fan dipole antenna plan and the "Ugly Balun" plan as well.

Good luck with the antenna.

73
Bob

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