So it has been forever since I’ve posted anything, let alone made any serious attempts at “playing radio” so I figured that I should change that! I’ve had too many excuses not to get this project done; time to make good on a promise to myself.
A project which I’ve been meaning to complete for a very long time is to place a dipole antenna in the attic space of our house. This will provide me a means of reaching out to the world while I save my pennies and look for a good deal on a multi-band Yagi-Uda HF antenna (and possibly a different tower too).
One of the troubles with any antenna system is connecting the antenna to the radio (or vice versa depending on your stance). Many types of feed line are available, some with distinct advantages but they always come at a cost!
Having purchased what is usually a very expensive form of coax: Heliax, I wanted to finally make use of the cable. This is a perfect place to use such a specimen – I need to situate my transmitter some distance away from the eventual antenna location, so losses become an issue. This is where something like Heliax comes in handy.
When running the feed-line from my mechanical room to the attic, I wanted to offer physical protection for the coax. Using a conduit would accomplish this task as well as provide a future path for other cables to be run in the future.
I was fortunate enough to get my hands on quite a long piece of scrapped liquid tight flexible conduit. My original plan was to use PVC tubing, but the FR4 rated conduit which is a plastic coated metal armor (similar to BX wire) offered a much more elegant solution.
I installed the liquid tight conduit from my mechanical room to the attic of the house, fire-sealing all structural penetrations to ensure maximum safety while meeting code for electrical work (which is not technically required in this case, but is a best-practice approach).
With the conduit in place, the next step would be to run the feed line. In this case, I used a portion of the LDF2-50 (3/8″) Heliax cable I purchased back in Oct 2014.
The Heliax will allow me to have my shack virtually anywhere in the house while experiencing only 2.26dB of attenuation at 440MHz for every 100′ of cable and a measly 0.383dB per 100′ at 14MHz – talk about awesome for coaxial cable!!
Getting the Heliax cable through the conduit proved to be quite a challenge. I have fished large cables through conduit, etc many times but this proved to be particularly challenging as I was working alone.
Over the course of a few hours, I made a few trips up into the attic and then back into the basement, trying to pass the fish-tape through the conduit. Each time I would encounter a new challenge which would have my patience being tested. The worst part is that I knew that I should have passed the pull line through the conduit while it was on the ground and in a straight line.
Never the less, once I had the fish-tape through the conduit, I pulled two mule-tape pull strings through, one to be used as a reserve. To prevent the pull lines from becoming fouled with each other, I hand-fed them into separate buckets to act as a pull bag of sorts – this is a rock-climbing trick and it worked like a charm.
As it turned out, having two pull-lines in the conduit made it impossible to pull the Heliax. Doh! I could likely have managed if I were to use a conduit lube but I did not want to use such a product for a few reasons, personal preference being the primary consideration.
After much cursing and nearly a hernia from pulling so hard, I realized that I would have to pull both lines out and re-pull a single mule-tape instead. This meant another series of trips up to the attic and down to the basement – a routine which was getting old pretty quickly.
This was a perfect example of a project where having a second person helping would have cut the time by more than half. There are so many tasks that you take for granted when you have someone to help you – a project like this is a great way to remind yourself of some of those challenges which are overcome with assistance.
I would have asked my better half but she was kind enough to watch our newborn son while I spent hours alone in the attic and the basement.
When it came time to pull the Heliax, I ended up spooling it on a surplus wooden wire spool that I acquired (with the intention of turning it into furniture).
The spool was necessary because as I pulled the Heliax, it had a tendency to start kinking up and I would have to make many tens of trips to check the cable as I pulled. This was simply not acceptable and a better method had to be developed, hence the wire spool.
Spooling the cable was extremely helpful – I would recommend this to anyone attempting something similar. I used a piece of 1/2″ black pipe to act as an axle and two tall automotive jack-stands to serve as a make-shift a-frame to hold the spool in place.
Once the feed-line was pulled through the conduit (which by the way would make even He-Man wince), I pulled enough slack to allow the cable to follow the ridge of the roof, all the way to the side of my garage.
This will be the future position of the Heliax feed-line once my tower is in place (presuming I do install it next to my garage).
For the current project, having the feed line above the antenna would cause all kinds of undesirable effects; one of which is a major distortion of the radiation pattern of the antenna.
I am not sure what I will be using for the feed-line for HF use in the future. I may very well use a pair of diplexers to take advantage of a single feed-line run. This is the lazy solution.
Being that I have more than enough Heliax to duplicate the same run, I may elect to run a second feed-line, taking the lessons I’ve learned from this exercise and applying them to any future work of this nature. This would allow for a multi-radio station without the worries associated with diplexer use: issues with guaranteeing at least 60dB of isolation between radios.
The dipole antenna I installed was cut roughly for 20m (14MHz) but the final tuning has not yet been performed, allowing for optimization for the in-situ configuration.
To facilitate installation, tuning, and future maintenance, the antenna supports were chosen to be ring-eyed lag bolts with pulleys attached. This method of connection would be more than strong enough for an antenna which will be protected from the harsh elements.
At either end of the dipole antenna, pulleys were once again used and lead weights tied to the supporting ropes to provide constant tension to the antenna system’s legs. Installing the antenna in such a manner will allow me to lower the middle section for tuning. In this case, the legs are shortened by loosening off the wire rope saddles and shortening the length of wire which attaches to the balun box.
It is possible to test-tune the antenna by folding the ends of the legs back on themselves but in my case, the antenna was built using copper weld wire and aircraft “anstad” type insulators which are apparently quite a pain in the butt to work with. I was advised by the gentleman who assembled this antenna that I would be best to tune from the middle so to speak. I picked this specimen up at the Carp Hamfest this past September as my G5RVjr did not hold up very well. I could certainly make an antenna but I tend to over-engineer things and don’t mind spending a little bit of money for something better built than I could manage with my limited parts.
For reasons of safety, I placed as much of antenna as possible away from the lived portion of the house. Canada’s Safety Code 6 outlines acceptable exposure limits for radio frequency radiation (at various frequencies) and it is important to consider safety when working within our hobby.
As you might see by reading over Safety Code 6, it does not take a whole lot of power to exceed the guidelines! Inverse square law and all, with an antenna less than 30 feet overhead, it is safe to say I would not be transmitting at very high power with this antenna system. My objective is low-power digital modes such as JT65 and QRSS.
The leg of the antenna which does intrude above the lived space does not penetrate very far (thankfully). All the same, such an antenna is most certainly a compromise and this needs to be considered when deciding on such an installation.
An attic installation was honestly not my first goal – it was born out of convenience and perhaps a bit of laziness. The whole project ended up being quite a bit more work than I had initially planned but I am glad to have it done.
Once the antenna was in place, I connected the feed line and fired up the spectrum analyzer so that I could have a quick peek at the return loss plot to check the resonant point of the antenna system now that it was in place. When an antenna is cut long, the resonant frequency will be lower than your anticipated frequency – this is extremely helpful knowledge when it comes time to make / tune an antenna.
As was expected, the resonant frequency was quite a bit lower than desired as the antenna legs were longer than the prescribed 16.63 feet for 14MHz (468 feet / 14.070 MHz / 2 = 16.63). This equation is only a jumping off point – it can not calculate the exact lengths needed for your installation as it does not consider the many interactions the antenna system has with its surroundings.
In the interim, I will be using my MFJ-902 travel tuner to provide a 50Ω impedance match from antenna system to the radio (FT-857D). This again is a compromise (for now) and will result in less than ideal performance but hey – at least I will be able to transmit now!!