# dipole

• posted

Antennas are new to me so the questions I ask may not be entirely sensible. But here goes anyway:

My understanding is that a dipole antenna is in the shape of a T where the length of each horizontal branch is equal to a quarter wavelength. So, for 2.4 GHz, each horizontal branch would be about 1.2 inches long. But I've seen articles and spec sheets that call the stock "rubber duck" antennas that come with many routers and client radios dipoles. They don't seem to be in the shape of a T at all. Rather they just seem to be a length of wire. What gives?

Would it be possible to build my own T-shaped dipole by soldering a couple of 1.2 inch pieces of wire on to the end of a coax cable? Well, it would be possible, but would it work ok and why or why not? :) Would I get more signal if I made the dipole a full wavelength long or longer? How about many wavelengths like a couple of wires running all the way across a room?

I know this borders on "turn your electrical wiring into a giant antenna" but, as a novice, I can't help thinking that I can "grab" a larger signal by putting up a bigger antenna.

Thanks, Bruce

• posted

Correct. Actually, it's more like 0.95 * 1/2 wavelength because of "end dispersion" effects.

Watch your accuracy. At 2400Mhz a wavelength is about 125mm. However, each MHZ is equal to: 125mm / 2400 = 0.052 mm/MHz The band is 83.5 MHz wide, so your overall tolerance on cutting the elements is: 83.5 * 0.052mm/MHz = 4.35 mm. it doesn't take much cutting error to end up with a non-functional antenna.

Well, there's many ways to make a dipole. In the case of the rubber ducky, it's called a coaxial antenna or vertical colinear. The antenna consists of a 1/4 wave driven element and a 1/4 wave sleeve fitted over the coax cable. If you built your dipole out of tubing instead of wire, and shove the coax cable feed down one of the tubes, connected at the center as usual, you would have a coaxial antenna. It's very cheap and easy to make out of just coax cable. It yields about 2.1dBi of gain.

However, if you disembowl some of the rubber ducky antennas, you'll find several different types and additions. The simple dipole is found in the antennas that are about 100mm long. The 200mm long antennas have an extra sleeve soldered to the coax cable braid about

1/4 wavelength below the feed point. This is intended to reduce VSWR and radiation from the coax cable feed. It also improves the gain slightly.

Yes. It's done all the time in feeds for dish antennas which are often simple dipoles. The antenna will be 75 ohms instead of 50 ohms but the mismatch loss in nominal. A balun (balance to unbalanced) transformer might be a good improvement.

It would work no better than the rubber ducky antennas. however, it could be used to position the antenna in a better location. Try to use fairly low loss coax and not junk.

No. Without phasing the additional 1/4 wave sections, you would end up with a pattern that vaguely resembles a cloverleaf at 1 wavelength per element.

What you should try is building a vertical colinear as in:

a b b b b a ===== =========== ========< >======== =========== ===== | | | | feed | | | | a | | a a | | a a | | a a | | a | | | | | | | | === === === ===

The long pieces (b) are 1/2 wavelength long. The stubs (a) are 1/4 wavelength long. The short cross pieces at the 1/4 wavelength stubs (a) are as short as possible. The end pieces are 1/4 wavelength long. The antenna can best be made by bending copper wire or brass rod. A sleeve balun might be used at the coax feed point if you want to squeeze every bit of gain out of the antenna.

It's not a perfect or great antenna but is very easy to build. You can expand it forever, but there's a catch. Doubling the size of the antenna only yields 3dB of gain. Most of the radiation comes from the two sections near the feed point.

A "long wire" antenna has problems with matching to 50 ohms. It starts to look more like a big inductor than a proper antenna. Getting RF to the end points of the antenna is difficult. Don't bother. Bigger antennas don't necessarily imply better antennas.

That's how I got my start in electronics in the 1950's. There was this crook in New York that was selling "Turn your House Wiring Into a Giant TV Antenna" kits. It had a "capacitator" inside and was rather dangerous with the AC/DC TV's of the era.

Nope. Think phase cancellation. If all the parts of your bigger antenna received the signal at exactly the same time, and exactly in phase, then you would have some gain. (Double the antenna gives 3dB gain). However, just a random wire antenna doesn't do that. Different parts get the signal at different times. That causes them to randomly cancel as well as add. I can model anything reasonable with an antenna modelling program (4NEC2). Methinks you'll find that long antennas for 2.4GHz don't work at all and are actually worse than a

1/2 wave dipole or rubber ducky. Think phasing.

If you want to build something, I suggest a Biquad antenna. This is basically two full wave loop antenna in parallel with a reflector.

• posted

Oh, I see. Like a T with the top turned 90 degrees. Thanks David. Still wondering if making the overall dipole length equal to a large numbe of half wavelengths will help.

Bruce

• posted

You're not wrong. The di-pole is both the centre conductor and the outer conductor, one going one way, the other going the otherway.

With the rubber duck antenna's what they've done is essentially wrap the coax back on itself by using a brass sleeve that fits over the outer insulation. In effect the T is still there but the tail of the T fits inside the brass tube.

Here are a couple of examples. Although I don't explicitly show the dipole but you can see where it enters, a parabolic:-

and the guts of a PCI card being modified to stick in a laptop.

Underneath the heatshrink sleeving is a piece of brass tube that is soldered inside to the coax braid, the centre conductor is then allowed to protude the 1/4 wavelength amount, 31mm.

Sorry, never got round to doing the page for the second one, just raw pics.

David.

• posted

Those are great ideas. Looks like I'll be bending wire all week :)

Thanks, Bruce

• posted

Yes, or if you make one half of the horizontal member of the T hollow, tuck the tail down inside it.

Jeff is much better on antennas, Jeff...

David.

• posted

A couple of years ago, Don Widder proposed the "simplest" antenna, but he specified 32mm for the sleeve and the exposed conductor. I haven't figured out how far off that is. I wonder if he's at the edge of the channel range, or maybe free space instead of coax, by mistake.

Once upon a time From Don Widders:

You can just make an antenna out of the end of the LMR195. Remove about 3 inches of the plastic outer 'jacket' of the coax. Then pull the copper braid back over the remaining jacket. Get a piece of brass tubing at a hobby shop that will just slide over the braid and cut a piece of the tubing to 32 mm. Trim away any excess braid. Cut the center conductor so that it extends exactly 32 mm from where it exits from the braid.

• posted

Yes, that's what I did with the miniPCI card conversion here:-

• posted

Oh, that's what _you_ did. I thought you had surgically removed the rubber duckie, exposing the existing internals.

Is Don's 32mm, verses your 31mm in this thread, verses your 30.5mm in the parabolic dipole, important? Why do yours differ by .5mm, or is that so miniscule as to be meaningless?

• posted

Nope, just unsoldered the wire which went to the SMA on the backplate and thought simplest thing to do was what I did. :)

Well I was rounding up but while I'm sure it's signicant to someone who is going to measure it, when in a laptop, you can probably cancel out the difference by turning the laptop around on the table!

David.

• posted

Yah, sorta. I tend to refer to a dipole antenna as a "half wave dipole" because of the overall length. The 0.95 is the calculated fudge factor based on the total length of the dipole. It is NOT a fixed number but an estimate. I need to know the wire diameter to calculate the exact value.

Ummm... I've seen worse.

It's difficult to define the length because I don't know the aspect radio. Tell me the wire diameter and I'll give you the free space numbers to about 4 decimal points. I usually cheat and just pound the numbers into an NEC2 modelling program such as EZNEC or 4NEC2.

A nifty trick to inscreasing the bandwidth of a driven element is to use conical shaped radiators or rounded ends on the rod. Most of my

900MHz rod antenna have rounded ends on the elements to improve the bandwidth. Fat elements also dramatically increase the antenna bandwidth.

The 0.95 is my guess as to the fudge factor for the overall 1/2 wave dipole. I don't know the magic correct number without knowing the construction details.

Try again. The above calcs are for a full wavelength. The necessary accuracy for a 1/4 wavelength would be 1/4th of that or about 1.1mm. That's 1.1mm difference to mistune the antenna over the entire 83.5MHz wide band. If you model the antenna and get exact numbers for building the beast, then to keep the antenna inside the 2400-2483.5MHz band, you need +/- 0.55mm accuracy on the 1/4 wave elements.

Another way of demonstrating the accuracy is: 1/4 wavelength at 2400.0 MHz is 31.25mm 1/4 wavelength at 2483.5 MHz is 30.20mm Therefore, the total allowed cut range accuracy is: (31.25 - 30.25) /2 = +/- 0.5mm If you want it to operate on a specific channel (1, 6, or 11), then the accuracy required is one third of the 0.5mm.

Can you say critical?

Bandwidth is directly affected by the antenna gain. Very roughly, for a given antenna type, a 3dB increase in gain (by doubling the size) will also double the Q (quality factor of the antenna) which effectively cuts the bandwidth in half. That means that fairly low gain resonant antennas (If that were applied to each 1/4 wave element, are you saying that the

Worse. +/- 0.5mm. See above calcs.

• posted

Isn't he speaking quarter wave and you half wave? He said quarter wave, and you counter with .95 * half.

Maybe I'm too fussy, measuring with a micrometer, marking with a piece of chalk, and cutting by biting the wire with my teeth.

My dilema: I see 30.5, 31, and 32 suggested, all of which seem to long, given a center frequency of 2437MHz and the .95 factor.

It would seem that the optimum would be 29.2mm for Channel 6.

299792.458 / 2437 / 4 * .95

Why, then, do I see suggestions of 30.5 (which would be 100% of 2400MHz, outside of the band), or 31, or 32mm? Where is the .95 applied?

A margin of error of 4.35mm on an element that is only 31mm long is obvious enough that I would hope people can get it right.

But that's a full wave, not the margin on each 1/4 wave element. It also suggests that such an antenna would be perfect for some frequency with the range of 14 channels, but doesn't say anything about the tolerance for an acceptable antenna that will be used specifically on channel 6.

If that were applied to each 1/4 wave element, are you saying that the margin of error on cutting the element is 1mm?

• posted

As I understand it, neglecting the .95, a dipole with each leg 1/4 wave long (1/2 wave total length) has an input impedance of 50 Ohms. A dipole with each leg 1/2 wave long (1 wave total length) has an input impedance of 75 Ohms. I'm using 75 Ohm RG-6 cable to keep costs down, so I'm probably better off with 1/2 wave legs. At least I'd be half right that way.

Cabling-Design.com Forums website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.