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N0ZLD
05-02-2006, 08:00 PM
All,

I am looking for help for a good length to cut a 40m base-fed Inverted L. I am unable to find any calculators for something like this on the internet. I will be using a Icom AH-4 to 'tune' it.

Also, is there any noticable difference between stranded or solid wire for this application? I was going to use stranded (speaker wire to be exact) for the antenna.

Thanks in advance,
Elijah

WB2WIK
05-02-2006, 08:14 PM
For 40m, a length that requires no tuner is 1/4-wavelength overall, which would be 33 feet (including both vertical and horizontal sections).

Since that's pretty short, the next length that requires no tuner is 3/4 wavelengths, or 98 feet. That's a good length. You can make it (for example) 40' vertically and 58' horizontally and have a reasonable inverted-L that should perform well. Of course, you could also make it 50' vertically and 48' horizontally if you have a way to do that.

Other lengths work fine, too. But good lengths to "avoid" are any integer of 1/2-wavelength: For example, 66' is not a good length, nor is 131 feet -- or anything even close to those lengths. I'd avoid any length from 56' to 75' long, or from 112' to 150'. Just too difficult to match, even with a decent autotuner.

How well it will work is highly dependant upon your radial system! 20-30-40 radials, all about 1/4-wavelength (33' or so) long, isn't bad.

Stranded or solid isn't important, but (most) "speaker wire" is usually intended for indoor use and won't stand up to UV radiation; thus, the insulation will usually fall off the wire within a year or so, leaving bare wire remaining. I like to use something a bit more "robust" than that. Even very good "antenna wire" is cheap, I wouldn't skimp on this to save a couple of dollars.

WB2WIK/6

W5DXP
05-02-2006, 08:17 PM
Quote[/b] (KC0RDG @ May 02 2006,14:00)]I am looking for help for a good length to cut a 40m base-fed Inverted L.
An inverted-L is simply a 1/4WL monopole with the top bent horizontal. For instance, you could make your 40m inverted-L 16.5 feet vertical and then 16.5 feet horizontal. It still requires a good radial ground plane.

Most hams can manage 33 feet of vertical monopole for 40m so one doesn't often see a 1/4WL inverted-L used on 40m. They are very popular on 160m.

N0ZLD
05-02-2006, 08:35 PM
Thanks for the replies! I am trying to come up with a 40m-10m 'all bander' using the AH-4 while living on a city lot and not having more than $200 to spend - thus a tower, push up mast or anything of the sort is out of the question. I do have a couple questions about the advice...

Why does it have to be (or why are you recommending) that it be cut for 1/4 or 3/4, 33 or 98 feet respectively? I would assume this is for maximum radiated RF? For instance what if I did a 25 foot vertical section and a 25 foot horizontal. Would it just make the 'tuner' work 'harder' (add more cap or induc, etc) and thus cut back more RF being xmitted?

Why are 1/2 wavelengths to be avoided? I assume this goes far more in depth in a technical way that what could be said here.. however, if you have a link, I'll read it http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

In all reality, I do not have a way to get the vertical section mounted more than 30 feet straight up (at the most extreme max) because I do not have a high support at where the tuner is and thus where the vertical section will extend from. The vertical section will be running 3 feet away from the side of the house up to a dormer on the house and then from there to a tree in the backyard.

I do not have money for a tower, push up mast or anything else. The budget is $200 and under.

K0RGR
05-02-2006, 08:55 PM
It's a matter of impedances.

A 1/4 wave antenna, working against some kind of ground, will have an impedance at the feedpoint of around 35 ohms, ideally. A 1/2 wave antenna, fed in the middle, will have an impedance of about 70 ohms in free space. But, if you feed that 1/2 antenna at one end, the impedance will be very high - possibly thousands of ohms. This makes it very difficult for most antenna tuners to handle. Odd multiples of a 1/4 wave antenna have similar impedances to a 1/4 wave antenna. Even multiples of a 1/2 wave antenna act like a 1/2 wave.

KL7AJ
05-02-2006, 09:19 PM
Actually, if you just cut the thing for a total length of 1/4 wave, it should be good enough for government work.

N0ZLD
05-02-2006, 09:33 PM
I think I may have asked to many questions in 1 post and thus they don't get answered. I'll single out one....

What if I did a 25 foot vertical section and a 25 foot horizontal?

KL7AJ
05-02-2006, 09:44 PM
Quote[/b] (KC0RDG @ May 02 2006,13:35)]Thanks for the replies! #I am trying to come up with a 40m-10m 'all bander' using the AH-4 while living on a city lot and not having more than $200 to spend - thus a tower, push up mast or anything of the sort is out of the question. #I do have a couple questions about the advice...

Why does it have to be (or why are you recommending) that it be cut for 1/4 or 3/4, 33 or 98 feet respectively? #I would assume this is for maximum radiated RF? #For instance what if I did a 25 foot vertical section and a 25 foot horizontal. #Would it just make the 'tuner' work 'harder' (add more cap or induc, etc) and thus cut back more RF being xmitted?

Why are 1/2 wavelengths to be avoided? #I assume this goes far more in depth in a technical way that what could be said here.. however, if you have a link, I'll read it http://www.qrz.com/iB_html/non-cgi/emoticons/smile.gif

In all reality, I do not have a way to get the vertical section mounted more than 30 feet straight up (at the most extreme max) because I do not have a high support at where the tuner is and thus where the vertical section will extend from. #The vertical section will be running 3 feet away from the side of the house up to a dormer on the house and then from there to a tree in the backyard.

I do not have money for a tower, push up mast or anything else. #The budget is $200 and under.
Those recommended lengths (overall) will give you a self-resonant system (within reason) and a relatively low impedance. 1/2 wavelength and multiples have a very high impedance at the ends, which will require a tuner of some kind, and even with said tuner, it tends to be touchy. 1/4 wave antennas are just "better behaved.' There is nothing, however, that makes a self-resonant antenna a better radiator...any random wire will radiate, and until you exceed 1/2 wavelength or so, "gain" is irrelevant.

The ratio of horizontal to vertical of an inverted L has little effect on the impedance.

Eric

WB2WIK
05-02-2006, 10:57 PM
25' vertical and 25' horizontal will work on 40m and is about 0.38 wavelengths (~3/8ths wavelength). That is a reasonable length and should be pretty easy to match for your tuner.

Remember the radials are more important than the length of the radiator, in terms of making an overall efficient antenna.

The reason you'd want to "avoid" certain lengths (like any multiple of 1/2 wavlength) is these are difficult lengths to match, even for a good tuner. Feedpoint impedance (resistance) is just too high. Leads to tuner arching and headaches, and is just best avoided.

Problem with "1/4 wave" antennas is when you go to use them on multiple bands. 1/4 wavelength is great for the primary band, but on the second harmonic of that band, it becomes 1/2 wavelength, which is not good.

If you're installing an antenna to be used *ONLY* on 40 meters, 1/4-wave is terrific. But if you want to use it on 20 meters also, it's a bad choice. It will work okay on the third harmonic (15 meters), but then 10m is a bad choice again.

For working multiple bands, especially harmonically related ones, it's often best to *avoid* a resonant antenna, and shoot of a non-resonant length. This is one reason why so many people use inverted-L antennas that are 165' long overall (maybe 40' vertical, 125' horizontal, or whatever). That's non-resonant everywhere. It's more than 1/4 wave on 160, more than 1/2-wave on 80, etc. Doesn't achieve resonance anywhere, which is a good thing, because that means it's never 1/2 wave or any multiple of 1/2 wave anywhere. Very useful length for a multiband inverted-L.

Remember the radials.

WB2WIK/6

N0ZLD
05-02-2006, 11:38 PM
So a 50 foot wire will work for 40-10 meters without causing issues on any of the bands whether 1/2 wave, etc.?

kd5rpo
05-03-2006, 03:37 AM
Quote[/b] (KC0RDG @ May 02 2006,17:38)]So a 50 foot wire will work for 40-10 meters without causing issues on any of the bands whether 1/2 wave, etc.?
Absolutely! That is what I have been using for years in this small city lot. As long as you have a tuner, there is an advantage to not using multiples of a quarter wavelength.

When you feed a quarter wave against ground you are feeding at a current maximum which means you will experience the highest possible losses in your ground system.

AB6ND
05-03-2006, 04:08 AM
What's the most difficult thing we hear about? A good rf ground. For years (over 50) I've used a simple end fed antenna 65 feet long. Yes a half wave on 40! Link coupled to a small coil and the antenna taken from the end of the coil away from the link. I don't have an rf ground and some 1/4 wavelength wires connected to the back of the tx seems to take care of "hot spots" around the shack. KISS
Used it in many locations with decent results.
AB6ND

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