Building a 5/8 wave antenna

[Cat Driver]

Can a person build a real true 5/8 wave antenna without having to use a coil or such at the bottom (the one inside) like the maco 5/8 does?
I own a small machine shop an was just wondering what it would take without having to sub out parts for one.

Ronnie B.
The Toolman

 

[W5LZ]

Toolman,
The only 'biggy' with a 5/8 wave antenna is matching it's feed point impedance with the feed line's (and then the transmitter's, of course. Just make it 50R and 0X and it'll work fine.). There are always more than one way of doing things, so you can do away with the 'coil' thingy. That would mean substituting something else that does the same thing as that 'coil', though. If you can think of, or find something else, then why not?
- 'Doc

(One method is by using the inductive characteristics of coax, involves a 'stub'.)

 

[Mr Clean]

You Can use a Q section for a match. Its a impedance transformer you can make out of two sections of coax tied together. Got the info from the Pratical Antenna Handbook by Joe Carr. Exelent book and comes with a antenna modeling cd. About 50 bucks but well worth it. Cover everything you need to know about antennas. Im sure you can find somthing about Q sections on the net

 

[FL Native]

all you have to do is slope the radils down to about 45 degrees.

then you wont need a matching network. you can also for this use a 1:1 balun, or a small dollar match air coil from main radiator to the ground side. without some type of matching devise you will have your coax radiating/CMC.

http://www.dxengineering.com/pdf/K7CIE Hustler doc.pdf

see page 5 for the small air coil.

now if you want the 90 degree radils.and the matching network. 6 foot loop of wire or tube from the main radiator to the ground radials, or trumbone like the I10k. then run center coax to the middle of the loop. coax shield to the ground radials.keep main radiator and ground system seperated. with a piece of fibreglass or what evr you want that isnt conductive.
ajust swr with the center coax on the loop.
102 - 104 on the ground radials. and around 22.5 feet on the main vert element.

heres a link. substitue the wood for fibreglass rod or a 4-6 foot shovel handle.

http://home.att.net/~wizardoz/cbmw/antenna_fabri.html#doit

about 1/2 way down that page. on that set up they still lower the ground radials. try it test it both ways.

 

[Master Chief]

Can a person build a real true 5/8 wave antenna without having to use a coil or such at the bottom (the one inside) like the maco 5/8 does?

Yes.

all you have to do is slope the radials down to about 45 degrees. then you wont need a matching network.

Nope, this is wrong.

 

[FL Native]

Can a person build a real true 5/8 wave antenna without having to use a coil or such at the bottom (the one inside) like the maco 5/8 does?

Yes.

all you have to do is slope the radials down to about 45 degrees. then you wont need a matching network.

Nope, this is wrong.

i siad about. so ok it could match up at 30 degrees. :roll:

 

[Captain Kilowatt]

FL Native ,the sloping radials work with a 1/4 wave verticle but not a 5/8 wave.Sloping the radials raises the feedpoint impedance of a 1/4 wave from around 35 ohms to 75 ohms depending on the angle.A 5/8 wave is far from anything close to what simply sloping radials will match up.

 

[Master Chief]

A 5/8 antenna is nothing more than a main radiating element that is 5/8 of a wavelength long.

Add ground radials to it and you have a 5/8 groundplane.

The trick is getting the feedpoint to match with the feedline. If you want to use coax, you need the feedpoint of the antenna to be 50 or so ohms. In order to do this you need to take the antenna to the next 1/4 wave, which in this case is 3/4 wave. This is accomplished by using a loop or coil to make up the missing 1/8 wave while trying not to interfere with the 5/8 radiating element.

If the main radiating element gets longer than .64 wave (or .625 for a 5/8 ), the optimum radiation pattern breaks apart and you get get lobes in all the wrong directions (like straight up). Its like popping the balloon!

I have a friend who lives up against a large hill. He wanted to mount his I-10K high on this hill, which put it over 500' away from his station. He bought Heliax to compensate for the loss. I told him we could feed his antenna with 450 ohm ladder line and his loss would be almost zero. This would require changing the feedpoint of the antenna to accept (match to) the 450 ohm feedline. We never did this experiment unfortunately.

So you see, the "ANTENNA" is the 5/8 radiator. The loops and coils you talk about are only a matching network for the feedline.

I have said over and over and over again......a 5/8 is a 5/8 is a 5/8. The "magic" of the I-10K is the low loss, very efficient matching network! The more energy you get in and out of your 5/8 radiating element, the better it performs! Everything works; some antennas work better!

A good example of better efficiency would be the differences between RG-58 and RG-8. Which has more loss? Would you rather run 100' of RG-58 or 100' of RG-8? Why? Answer that and you just might understand why antennas with lower loss in the feedpoint work better.

Then there are the mechanical differences (quality materials and build strength), but I'll save that for another thread.

 

[lords]

I have a friend who lives up against a large hill. He wanted to mount his I-10K high on this hill, which put it over 500' away from his station. He bought Heliax to compensate for the loss. I told him we could feed his antenna with 450 ohm ladder line and his loss would be almost zero. This would require changing the feedpoint of the antenna to accept (match to) the 450 ohm feedline. We never did this experiment unfortunately.

dam, wouldn't mind hearing about that

 

[FL Native]

FL Native ,the sloping radials work with a 1/4 wave verticle but not a 5/8 wave.Sloping the radials raises the feedpoint impedance of a 1/4 wave from around 35 ohms to 75 ohms depending on the angle.A 5/8 wave is far from anything close to what simply sloping radials will match up.

hmm ok ........ thought it worked on both. maybe i read something wrong some were.
on this interneck.

ok if that is the case. why do some 5/8th angle them and some use 90 degrees?

what is the difference on a 5/8th with them at 90 and 45 ?

does it change the near and far field ? angle of take off ?

 

[Master Chief]

why do some 5/8th angle them and some use 90 degrees? what is the difference on a 5/8th with them at 90 and 45 ? does it change the near and far field ? angle of take off ?

Let's find out:
http://www.cebik.com/gp/58-3.html

The 10-meter patterns show once more that the use of sloping radials is marginal or detrimental, just as it was in the VHF region when the antenna was several wavelengths above ground. The flat-radial monopole has the greater signal strength at the lowest elevation angles. However, what may be more important is the fact that for either case, the poorer the ground, the better the antenna performance, both in terms of gain and in terms of pattern shape.

 

[Captain Kilowatt]

Master Chief has the answer.

 

[FL Native]

ya i read it. i see the differnce now. looks like if you lived in a valley you would want to sweep them down.

still dont understand why it only changes the match on a 1/4 wave and not on a 5/8.

 

[Marconi]

The typical condition at the feed point of a 1/4 wave vertical element with the GP radials horizontal is low impedance (25-35 ohms), high current. GP radials are effective in a high current condition and thus GP radials are very important in a 1/4 wave. Lowering the radials down tends to raise the feed point impedance a bit.

The condition at the bottom of a 5/8 wave vertical element is pretty much the opposite (>100 ohms) and GP elements likely won't have much current flowing in them because the condition at the feed point is a high voltage, high impedance, a low current area and needs some form of matching.

 

[W5LZ]

Here's a different way of looking at it.
Start with a 1/2 wave vertical dipole. It's input impedance is something around 75 ohms. As you start to raise the bottom leg (turning it into a radial) the feed point impedance starts to decrease. When that bottom leg gets to about 90 degrees from vertical, it changes the input impedance to something around 30 ohms.
Okay, same thing with a 1 1/4 wave vertical dipole (5/8 wave on each side of center). It's input impedance starts at around 40 ohms and 100ohms of reactance (40r - 100x). As you start raising that bottom leg the input impedance starts to decrease just like the other example. When you get to where the bottom leg is about 90 degrees from vertical the input impedance is something on the order of <10 ohms and can't remember what the reactance value is, not good anyway. So, the degree of 'tilt' on the radials does tend to change input impedance. If that's good or bad just depends on where that impedance starts from. With a 5/8 wave antenna the 'starting' impedance isn't anywhere close to where it can be 'fudged' by just tilting the radials. You have to do that 'matching' of impedances by some other method. Those 'other methods' include both capacitive and inductive reactances, you can do it with either a capacitor or a coil. Since coils are a bit easier to work with, I'd rather do it that way. Which ever way 'turns you on'...
- 'Doc