16 el coaxial colinear (2.4ghz) question

I salute anyone who has the patience to build a 16 element coaxial collinear.

Tr to visualize the radiation pattern of the antenna as a donut. The more elements you add to a collinear, the more the donut gets squished. Because your antenna has altitude, you may be sitting beneath the idea location of the donut. There are tricks to get downtilt in verticals.

You also need to consider the loss in the feedline.

BTW, I've seen coax colinear designs that aren't totally made out of

Ken Bessler wrote:

"Ken Bessler" hath wroth:

I wish my measurements were accurated to so many signifigant figures.

Show us a photo of your construction.

Yep. Just about everything about that design is wrong. Compare it with a proper construction article:

1. Each sections is 0.5 wavelength (electrical), not 0.25 wavelength. Only the very top element is 0.25 long.
2. Low loss LMR-400 is used, not high loss RG-58 junk coax.
3. The base section includes a dual 1/4 wave sleeve balun to prevent the coax from radiating and to keep the radiation angle from going up.

However, even if you build such an antenna, there are problems.

1. The alternating coax vertical collinear is twice as long as necessary for a given gain. Every other 0.5 wave section does not radiate and only acts as a phasing section. Since only every other element radiates, it ends up twice as long. A better design would be to use inductors or stubs for phasing, resulting in a smaller antenna. Something like:
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   Incidentally, the 0.5 wave section design is not the only way to build a vertical collinear. See:
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2. High gain vertical collinear suck because most of the RF comes out from the first (bottom) section. The way it *ROUGHLY* works is half the power radiates from the first 0.5 wave section. Half of what's left radiates from the next section. Half of what's left radiates from the next section, ad nausium. By the time you get to the top, there's very little radiation. In addition, the use of coax cable adds some attenuation. For fixed tower use, the addition of ground radials often raise the radiation angle above the horizon.

Jeff Liebermann hath wroth:

Oops, I goofed. The RG-58 design is 0.5 wavelength (electrical) sections.

This version is a bit better:

I've built plenty of VHF and UHF super J-poles and wondered two things. One, why can't the super J-pole be adapted to wifi. Is it the lack of small diameter copper pipe? Two, the coaxial collinear, being something like a half wave, still would benefit from some sort of ground plane, yet this is never mentioned in the designs.

snipped-for-privacy@sushi.com hath wroth:

I previously itemized why such antennas are not particularly wonderful. However, I have built them in the distant past and added a few mistakes of my own. I built a 444Mhz version that was almost 20ft long. Suspended from a tree, it seemed to work well enough. VSWR was low and it did have some gain (which I couldn't measure at the time). Satisfied, I shoved it into a 1" dia PVC pipe and filled it with urethane fence post foam compound. Suddenly, the antenna didn't work as well. The PVC and foam combination have what I guess to be a velocity factor of about 0.95. That moved the resonant frequency down, raised the VSWR, and ruined the antenna. Instead of a 444MHz antenna, I had a 430MHz antenna. I used a Dremel tool to slice the pipe open, extracted the antenna, and used a fiberglass tube instead. That worked.

Agreed. I like biquads. They're easy to build, fairly non-critical, and offer reasonable 9-10dB gain. My favorite biquad:

A j-pole antenna (inverted Zepp) is a fairly lousy antenna. It's major advantage is that the entire antenna is at DC ground thus avoiding messy insulators (and nasty sparks for Zeppelin gas bags). It's also easy to build out of pipe. On the negative side, it has a fairly narrow bandwidth and a high angle of radiation if mounted upwards instead of hanging downwards as in the Zeppelin.

A J-pole would probably scale nicely to 2.4Ghz. Fat copper pipe is not needed. The reason one uses fat pipe for 50MHz and below is to increase the bandwidth of the antenna. The larger the OD, the wider the bandwidth. At 2.4GHz, the equivalent diameter would probably be a #10 awg solid wire. The feed point gamma match could be made from the traditional wire wrap, but methinks a very small trimmer or fabricated tubular capacitor would be better.

Benefit how? The purpose of the ground radials is only to stabilize the impedance of the antenna. Without the ground radials, the antenna will be seriously influenced by the mounting structure. With the ground plane, the impedance of the antenna is stabilized by the "artificial" ground.

In the case of 2.4Ghz antennas, one only needs to be a few wavelengths above the mounting structure to be considered "far" away. Mounting will have an influence, but not as severely as at lower frequencies, where the mounting structure becomes part of the antenna. The dual sleeve decoupler at the base provides the necessary isolation and ground plane.

See:

Jeff Liebermann hath wroth:

Speaking of measurments, ever wonder how accurately one has to cut such an antenna? Grinding the numbers...

At the low end of the 2.4Ghz band, one wavelength is: 3*10^10 cm/sec / 2400*10^6 cycles/sec = 125 mm At the high end, it's: 3*10^10 cm/sec / 2483.5*10^6 cycles/sec = 120.8 mm

The tubes are a half wavelength long, times the 0.66 velocity factor resulting in 41.6 mm and 40.2 mm for the band edges.

If you aim for the middle of the band, with a nominal cut length of

That level of accuracy is impossible to achieve with coaxial braid, which is why the article at:

Ah, but the super j-pole is another story. The collinear again tends to squash the donut. People love or hate j-poles.

My point is the construction at 2.4Ghz is actually hampered by the small dimensions. Half inch copper pipe would make the phasing element in a super J-pole somewhat nebulous.