Legal Limit L-Match Tuner
The 40M loop is an excellent antenna but its very nature, it's limited to 40M and the higher bands. I really wanted an antenna to cover the 160/80/60 meter bands. I don't have that kind of space to erect even a dipole or vertical to cover these bands - or do I?
It's possible to convert the 40M loop into a kind of "fat" vertical by shorting the ladder line and working it against a ground system. The first challenge was installing a ground system. I have a few 500' spools of 18ga wire, so I ran a few "radials" around my property. I attached them to the station 8' ground rod, house power ground, and the cold water pipe out front. This wire also goes around the perimeter of the foundation. It's temporary for now, just enough to attempt a proof of concept. Using a MFJ 16010 and an antenna analyzer and was able to get < 1.5:1 match on the bands of interest. With that done, I started the design of a higher powered unit. Most of these parts I've had since I was in high school! About time I put them to use.
The match is built around a 35 uH roller inductor and 350 pF variable capacitor. I needed an antenna switch too, so found a high current switch in my junk box. I've been working on a process to create good looking front panels. I researched silk screening and that didn't seem too difficult to do. It's just a messy process and pretty much all or nothing. If the mask is off-center, you have to start all over. The process I developed uses a HP Photo inkjet printer with Costco photo paper. The paper I used is similar to this but what I found was 150 pages for $18. Not bad. I really like black panels and decided on red lettering. Here's my first attempt at designing the front panel.
This photo was taken a few days after it was printed. Yes, that's "black". What I found out was pure black won't stay that way, it ends up turning a very dark green. I liked the layout though, roller inductor on the left, antenna switch in the middle with variable capacitor on the right. All of these parts are probably 60 years old, maybe more. After some thought, I decided to use a gray background with black text. I don't know why black text doesn't turn green, but it stays black. I use Inkscape to lay out the panel. In order to make the dials, there's a dial extension for Inkscape to make any kind of linear or curved dials. I print the panel out onto photo paper with the printer set for photographs. With this particular project, the tuner is 17" wide so I couldn't print the entire panel out in one print job. I split the front panel into thirds, or thereabouts. I'll write up a page on the details of how to create front panel labels for your projects that are easy and cheap to do.
The front panel is made out of aluminum; I used a step drill and hand punch to prepare it for installing the inductor and switch. I used RG-8X coax with teflon dielectric for most of the internal wiring. The white wire is just the center conductor covered with dielectric soldered to the roller inductor, the other one on the right will get connected to the back panel output to the ladder-line. The grey coax will be connected to panel mounted SO-239 connectors. The variable capacitor will mount inside the case, with the shaft extending through the hole on the left. You'll see another hole, that's my goof hole and every project requires one in order to have a successfully working project. No worries, the panel label will cover it up.
Here's the front panel and such slid into the case. The case is a recycled shipping pallet. I grabbed a couple very high quality pallets from work that would make great cases. I sliced out a section using a table saw, then spray painted it flat black. Real easy, looks good, and free is always a great price.
Here's the completed L-match. Even using a good paper trimmer, it was nearly impossible to get the seams between each third of the panel to not have gaps. With the proper application of electrical tape, the seams are pretty much hidden. It does look rather industrial but that's OK. It's my first project using the front panel process and had some learning curve to flatten out.
The rear panel isn't completed yet. I need to order a few more connectors and I'm waiting on a delivery of a high current knife switch. I'll update this page once this is finished.
The tuner is installed after some re-cabling and ground cleanup. This tuner is the main RF connection and ground point for the station. On top of the L-match is the Balanced Line Tuner. The BLT setting on the switch changes the loop to ladder-line fed 40M loop which works 40M-10M. When the output switch is in the WIRE position and the ladder-line is switched to the WIRE port on the back of the L-match, the loop is now working in vertical mode, against the "radials" mentioned previously, giving me 160M-6M sans 30M. One antenna to rule them all! Big question is, how well does it work?
After installation and creating a tuning chart, I dialed up a station on 40M. Made a quick tail-end call to N3APA near Napa, CA. Solid S9 running 100 watts into vertical mode. APA was a good 20 over 9 but he was running 500 watts into a G5RV at 60'. So far so good, but how do the two loop modes compare to each other?
Around 0000z, Voice of Russia starts their North America broadcast on 7250. Using the loop in loop mode tuned with the BLT, I can barely hear their signal. This is good because I can use the vast majority of 40M and not be bothered by the shortwave stations broadcasting at the top of the 40M band. Switching the the loop to vertical mode using the new L-match and VoR jumps to 40dB over 9! Well, that was a total surprise. I was expecting an increase in signal strength but not that much. VoR is reported to run 500KW during this broadcast. So, just how long is this vertical?
I got to thinking about how "tall" the loop is when in vertical mode. The loop is 43' per side then fed with about 30' of 450 ohm ladder line back to the tuners. I'm thinking that the loop is roughly 90' tall if I take the hypotenuse of the loop as the straight line "length", then add the 30' of shorted ladder-line to that. Being 1849 sqft, I'm sure the antenna has some capacitance when worked against the ground system. This an aerial view of the loop antenna and existing ground. Loop is in red, existing ground is in white. The plan is to replace the white prototype ground with black, teflon coated wire. I bought 1000' of black 18ag wire at the swapmeet. The plan for the permanent ground system is in green. The aerial view of the ground system looks like this:
I'll be able to bury radials across the front and back yards when we do new landscaping this spring. All in all, even with a small suburban lot, a 40M loop is proving to be an excellent, easy to build performance antenna.