antenna-theory.com

[M1OzFan]

A 1/4 wave vertical monopole is often an attractive option as long as the usual ground plane is easily arranged. Where the simple flat plate is unacceptable, would a section of tubing, colinear with the monopole be suitable? At first sight, the result is a vertical half wave centre-fed dipole. The new element could just be part of the support tube (likely about 3m high in total). Is this a silly question? Is the answer an utterly simple "yes"?.

[admin]

This is totally ok

When you discuss 3m though, it is helpful to understand what frequency, otherwise hard to know how big or small the distance really is in terms of wavelengths.

[M1OzFan]

Thank you. Frequency range is 700 - 800 MHz so 3m height is not difficult. With a good insulator between the end of the lower element and the rest of the support tube, it all seems straightforward.

[M1OzFan]

If only life were as simple as converting a monopole to a dipole! With some more enquiry, the starting point is not a simple 1/4 wave monopole. It's intended for a wider frequency range, 690 - 960MHz and 1710 - 2610 MHz (4G in APAC region) and likely to be a two-section vertical collinear array (for advertised 5dBi gain). Element length for these is typically 1/4 wave or 1/2 wave coax (vel factor 0.66?) for the frequencies, what's a wavelength? Development was probably trial and error to deliver a reasonable VSWR and gain. Vertical collinear array usually needs a ground plane but we don't want to use a horizontal element. Can antenna theory help with the question of whether a vertical aluminium tube below the antenna can replace the ground plane? Is trial the error the only way?

[admin]

Probably watch this video for some understanding of dipole antennas and what to do to make the 2nd harmonic radiate:
https://www.youtube.com/watch?v=RF5r64fmFhU

Also, all cellular antennas in phones do the frequencies you mention, so if there's any resources on teardowns online it may also give you some ideas there.

If your dipoles are in air your velocity factor is 1. For the freuqency of interest, the wavelength is always c/f. Don't make it complicated

[Galoxlos]

Thanks for the info

[R. Fry]

Below is a NEC4.2 analysis of the current and phase distribution for 750 MHz along a 3m, center-fed dipole, and the resulting radiation pattern.

Even at the center of the lowest-frequency band mentioned in earlier posts, this antenna configuration is much longer than 1/4-wavelength, and consequently its performance may not be as expected for a "1/4 wave vertical monopole."

Using this configuration on the higher bands mentioned will produce radiation patterns with even more lobes/nulls than on 750 MHz.

[DrakenJ35]

Thanks for the explained thread, I'm a newbie and wanted to build an antenna in the 700-800 mHz wavelength as well, I think I'll be experimenting with an 1/4 wave monopole as well, and will see what I get from it (if I do it right, that is).

[belinda1]

This is totally ok

When you discuss 3m though, it is helpful to understand what frequency, otherwise hard to know how big or small the distance really is in terms of wavelengths.