Sunday, January 8, 2012

Hamradio Homebrew 2 Meter Square Dipole Plan

Here is a plan for homebrewing a 2 Meter Square Dipole plan. The advantage of this antenna is that it is unidirectional, and it takes less space than the regular 2 meter dipole. The calculation included on the diagram below is for building the antenna using copper tubing, you should use MMANA-GAL or other antenna simulation software to come up with new dimension for other materials (aluminium, wire, etc).
2 meter square dipole plan2 meter square dipole plan
Click on the diagram to enlarge it. Hopefully this will help you in brewing new antennas! Original plan taken from KOFF website

Antenna In Less Space – The Inverted Vee

Another space saving installation method is to mount the antenna in an inverted vee configuration. This type of installation has worked well for me over the years and I highly recommend it for those of which enjoy HF Radio and are in a limited space situation.

Unfortunately, there is no such thing as the perfect antenna. There are just too many variables for that to be possible. So the theoretically perfect antenna just doesn’t exist.  Ham Radio is all about trying out new things, and homemade antennas are no exception.

Experimenting with Ham Radio Antennas and using your creation to talk with other Amateurs all over the world is definitely one of the most accessible and enjoyable aspects of the Amateur Radio hobby.

Short Ham Antennas For HF

For many ham radio operators, it is not feasible to put up a a full length dipole on HF.
But, a linearly loaded dipole just might fit in your available space!
A linearly loaded dipole, as illustrated below...
  • ... is about 30-35% shorter than a "classic half-wave dipole" at the same frequency of resonance!
  • ... has a radiation resistance around 35 Ohms. (You will need an impedance matching tuner at the other end of the coax!)
  • ... just as effective as a "full length" half-wave dipole! :-)
Linear-loaded short ham antennas for HF.
Note that it "looks" like a folded dipole, but the top part is open! You should add a ceramic end-insulator in the opening, to add mechanical strength. (The insulator is omitted in the drawing to make the opening obvious;-).

Shorter Dimensions!

Here are "ballpark" dimensions for a linearly loaded dipole for each ham radio band. These dimensions are intentionally slightly long! You will have to obtain the final dimensions experimentally, on site, by "pruning" to resonance.
Forgot the technique to prune a dipole to resonance? Refresh your memory on the page about the ham radio dipole!
Linearly-Loaded Dipole
Approximate Dimensions
10 meters (28.5 MHz)
12 meters (24.9 MHz)
15 meters (21.1 MHz)
17 meters (18.1 MHz)
20 meters (14.1 MHz)
30 meters (10.1 MHz)
40 meters (7.1 MHz)
80 meters (3.6 MHz)
160 meters (1.85 MHz)
3.5 m. (11.5 ft.)
4.0 m. (13.2 ft.)
4.73 m. (15.5 ft.)
5.51 m. (18.1 ft.)
7.08 m. (23.2 ft.)
9.89 m. (32.44 ft.)
14.06 m. (46.14 ft.)
27.74 m. (91.0 ft.)
53.97 m. (177.08 ft.)

Construction Of Linear Loaded
Short Ham Antennas

I use commonly available 390 Ohm "ladder line" with #14 stranded, copper-clad conductors. It is sturdy and lasts for years.
For the central "attachment" I use two LadderLoc center insulators, head-to-head (available at Radio Works).
I recommend 3/16 in. Mil Spec Dacron® rope to tie the ends to tall supports such as trees.

Bonus Configurations!

Linear-loaded short ham antennas do not have to be limited to horizontal installations!
You can save even more space by installing them as "slopers", inverted "V"s and inverted "U"s.

Free Yagi Antenna Designs for Ham Radio

Some months ago, Bent OZ1CT asked me for a X-pol LFA (two LFA Yagis, one in the vertical plane, one in the horizontal plane on the same boom). Whenever I have received such requests I have been reluctant to work on them due to the many complexities involved in not just getting the model working but getting it working correctly and as model.
There are designs from many well-known designers and commercial entities around today but none that I have seen get it right in the electromagnetic design or mechanical construction and this in part is due to the lack of understanding of off-sets (elements) and the impact certain materials will have upon this antenna and without doubt, this is the main reason many hams are not 100% happy with their X-pol arrays.

I have spent much time modeling X-pol array in EXACTLY the arrangement they will be in once built, established the issues and made every effort to avoid and reduce any distortions. The details and result fo these findings will be published within the pages of DUBUS a little later this year but for now, take a look at one of the first ever X-pol LFA Yagis at OZ1CT.