If you’ve read any of my other posts, you know I love to build, tinker, and hack at stuff. Antenna-building is something I’ve not made a foray into, until recently.
I have a dual-band handheld radio for the 2m and 70cm bands. The so-called rubber-duck antenna that comes with it performs ok but it isn’t ideal. I could get into my local repeaters which are about 10mi away with enough power to break the squelch, but my audio was weak and quality was poor. Naturally, I decided I should put up an antenna!
To Build or to Buy?
You can buy antennas on the web or at local shops like my local (and fantastic) GigaParts, however in true geeky-nerd style, I have to DIY an antenna to feel I truly understand what is going on.
I’ll be the first to admit, prior to starting this DIY antenna journey, I didn’t have a great understanding of how antenna theory related to an actual antenna. I understood the general 1/2 wave, 1/4 wave relation to the designed frequency of an antenna, but until digging in, I didn’t get the math behind it. I found this website to be invaluable for a plain-language (plain to someone with a bit of an engineering tilt) introduction to the core concepts and how to apply them.
What kind of antenna should I build?
This was the next question I needed to answer. My antenna only had a few real requirements, namely:
- Support the 2m band
- Support the 70cm band
- Be outdoor-mountable
- Be home-manufacturable
This list ruled out a standard di-pole, because although I have learned you can “load” a dipole to make it multi-band, I didn’t feel I had the mounting capability at present for that. I also ruled out a ladder-line j-pole, because I couldn’t find ladder-line anywhere near me. It used to be more common I suppose when OTA TV used it for their antennas, but no more. I ruled out a Yagi also, based on the complicated layout, and I didn’t necessarily want a directional antenna. After scouring the web, I settled on a standard J-pole.
J-Pole: what is it and how does it work?
If you navigate to the Wikipedia entry for J-pole antenna , you will notice there are many “form-factors,” but they all follow the similar J shape, hence the name. But looking at the typical design, I couldn’t understand how the feed point (which separated only by a short distance) doesn’t just ruin the antenna performance. Take a look at the image below, credit ZyMOS.
I found the most useful explanation in a YouTube video (an aside: you can anything on YT these days) which I will link.
How it works, condensed version
A little prerequisite knowledge will help the understanding of how this antenna works.
- The impedance in the middle of a 1/2 wave dipole is very low
- The impedance at the ends of a 1/2 wave dipole is very high
- The impedance somewhere between “very low” and “very high” is our magical 50Ω
- We can end-feed a 1/2 wave antenna, but the impedance is very high (as seen in item #2)
Basically, the bottom U part of the j-pole is where the feed-point is adjusted to 50Ω (or thereabouts), and at the ends the impedance is very high, which is perfect to end-feed our 1/2 wave antenna (the long part of the J). As a result of this configuration, it really doesn’t matter which “leg” of the antenna is tied to shield vs. conductor for the feed-point.
But this is still for a single band, right?
Yes, but conveniently, the “long” side of the j-pole for 70cm band is pretty darn close to the “short” side of the j-pole for the 2m band, and since it doesn’t matter which element is our “radiating” element, we can essentially reuse one of our elements. See the rough diagram below for an example of this.
Let’s Build It!
So I had armed myself with the knowledge (at least the theory) and now I just needed to create a plan. Like any good engineer, I looked around the web for any prior work in this area (no reason to reinvent the wheel!) and much to my delight, someone had posted a PDF of an easy build process for such an antenna. Following this guide, I was able to construct my first dual-band j-pole.
Tuning the antenna
This step is very important, and can be made easier by the use of SWR meters or in my case a Vector Network Analyzer. I purchased the NanoVNA (wonderful tool, works like a charm) and was able to adjust the elements of my new antenna to achieve a great SWR and feed-point impedance in the 2m band, and pretty good SWR and impedance in the 70cm band. See captures below.
2m Tuning Results
70cm Tuning Result
Some notes on the 70cm performance
You’ll notice the oscillatory nature of the SWR and impedance in the 70cm band. I do not completely understand this, and may return to tweak the antenna a little more. I get decent performance in several “buckets” of the 70cm band, so it’s workable, but not ideal. I have a couple of theories as to why this is, but I don’t know for sure. If any readers have some comments on this, I’d love to hear it, please comment or send me some email!
The elements in the 70cm legs of the antenna are 3/8″ rod as opposed to 3/4″ EMT conduit. I suspect that the deep V around 441MHz is the center frequency for these stubs, and the antenna just has a narrow bandwidth because the elements are small. I will likely try to build another one with all 3/4″ elements to see if the antenna is improved.
I’m not sure that this has an impact or not, but the 3/8″ rod is threaded at 24-TPI and the 3/4″ elements are smooth EMT conduit. I wonder if this interplays with the skin effect?
The End Result
I mounted this antenna up on the roof, and boy-oh-boy does it work great! I am able to be heard at least 17mi away by an APRS digipeater, which was not possible with the rubber duck antenna. I can now full-quiet the local repeaters and my voice is loud and clear. Also, mounting the antenna outside has greatly reduce computer and monitor based interference, which was a pleasant surprise. All said, I’d say this was a great learning experience and fun project to boot!
Thanks for reading!