G8OO USEFUL INFORMATION 22/11/2018

 

A very usefull Dipole Calculator by the Fists Club (Click Here)
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MORE USEFUL INFORMATION By

AUTHOR: STEVE NICHOLS G0KYA - THURSDAY 29 JANUARY 2009

 

Multi-band loft-mounted dipoles for 40, 20, 17, 15, and 10m

 

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Amateurs are always after the Holy Grail - an antenna that will work on all bands, is inconspicuous, effective, but above all inexpensive. Unfortunately, you seldom find something that fits all these factors.

This antenna has nothing new about its design, but it does bring together quite a few useful ideas and characteristics.

These are:

Fully no-tune antenna system for five popular HF bands

Suitable for SWLS, QRP, M3 licensees and PSK31 operating plus occasional use up to 100W, but watch out for RFI at high powers. DO NOT USE A LINEAR!

Uses non-inductive (zig-zag) loading for 40m

Steve Nichols G0KYA

 

MORE USEFUL INVERTED DIPOLE INFORMATION BY,

AUTHOR: FREDERICK R. VOBBE, W8HDU, JANUARY 4, 2008

 

A half wave dipole in free space has a length calculated by 468 being divided by the frequency in megahertz (MHz). So let's suppose we are calculating for 7.1 megahertz, take 468, and divide it by 7.1, and you get 65.915492. Ok. I think we can round that to 66 feet. BUT WAIT. That's the ideal length.

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468 ÷ 7.100 = 65.915 say 66ft ÷ 2 = 32.95 say 33ft each leg

Add a about 6 inches to each end of the wire, then you can "tune" the antenna dead on the frequency by trimming back the wire as needed. In my 40+ years of playing with an antenna like this I have yet to calculate one that has matched on the first try. I always find that I have to add or subtract about 4 inches from the calculated length, so make your wires long, then fold them back to where they need to be.

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The inverted dipole is a very popular antenna. It's also known as an Inverted "V" antenna. The inverted antenna is a horizontal dipole with the ends tilted down to the ground. The angle between the two legs us usually between 90 and 120 degrees. A typical horizontal dipole for 40 meters would look like this;

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Due to the effects of the elements against each other, as well as ground, it's always been a practice to shorten the length of each wire by 3-6 inches. (More for lower frequencies than higher). In our example we can see that each 1/4 wavelength wire in our horizontal dipole is 32.9' long. But when designing it as an inverted dipole, on 7.100 megahertz, it was necessary to shorten it by 3".

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The dipole ends are usually, at minimum, 24" above the ground. Having the ends close to ground causes impedance artefacts and degrades performance. When installing this antenna it would be handy to have a VSWR meter or a way to measure the power reflected or the impedance of the antenna. In practice, the antenna feed point impedance will drop non-proportionally as the wires get closer to each other or ground. An installation at less than 90 degrees from each other is not recommended. Usually a better match is made with a 50 ohm transmission line rather than a 75 ohm transmission line. And keep in mind that the transmission line should be multiples of 1/2 wavelength at operating frequency. The performance of the inverted dipole is spread out, and less directional than the conventional horizontal dipole. However, gain is reduced. And due to the characteristics and interaction with ground, this is a very good antenna for 40 and 80 meter communications. Here's a bonus! Using insulators on your tower, and a high-grade "aircraft cable" wire, you can actually use this antenna as guy wires!

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Coming off the tower, place an insulator 6" off the leg. You will need about 18" of wire from a typical tower leg to the egg insulator. The wire for the antenna is then attached to the insulator, and goes toward the guy anchor, however, where the wire ends you place another insulator, then a wire that goes to the anchor. See figure 3. This can be done on 2 guy wires, or if you are innovative, do this with all three guy wires, and place a switching box up at the feed allowing you selection of any of the wires, which may favour a direction or null an offending signal. For example, if you have three wires, 1, 2, 3, you could use 1-2, 2-3, or 1-3. If the tower is high enough, you can also add a second level of inverted dipoles for up to 6 antennas! Also on a long guy run, it could be broken in two parts, with a trap in the centre, so you have the top guys wires being (3) 40/80 meter inverted dipoles, while the bottom guys are (3) 20/40 meter inverted dipoles.


Frederick R. Vobbe, W8HDU

 

G8OO USEFUL INFORMATION 22/11/2018

 

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