Inverted-L Myths and Realities

(Originally published in the Hampden County Radio Assn.‘s ZeroBeat, March, 2018)

The venerable Inverted-L is the most popular antenna for the low bands, due in large part to its simplicity. It has enabled many hams to get on 80, 160, or even lower from their city lots. Unfortunately, its ease-of-use has allowed substantial misunderstandings as to design theory.

This article will address several of the most oft-repeated myths regarding Inverted-L’s for the low bands. In a future follow-up article, I will detail the construction of a 160m Inverted-L at my new QTH using the “Ten Commandments” provided below.

Myths and Realities:

  • “I feed my Inverted-L directly and my SWR is great.”
    If you feed your inverted-L without any type of matching network but you have low SWR, your antenna is probably very poor. The low SWR is due to tremendous ground losses near the feedpoint. As you improve your radial system, SWR will actually rise and will likely require additional capacitance at the feedpoint. SWR is a poor design metric.

  • “Radials reflect your signal.”
    Your radial field provides a return path for RF (similar to the shield side of a dipole), but does not “reflect” your signal. The actual reflection happens several wavelengths away from the antenna and is due to something called the pseudo-Brewster Angle.

  • “This is a great limited-space antenna. Four radials should be fine!”
    How many radials do I need? Bad news: you need a bunch. For our poor soil conductivity, you’re going to need at least thirty and they should be ¼-wave long. I’ve found the length to be less important than the density near the feedpoint; for this reason, try to keep them evenly spaced, even if they are shorter in some directions. If you are extremely space limited, you can add a galvanized ground screen around the feedpoint (in addition to as many radials as possible, as long as possible). Good news: 30 radials appears to be the point of diminishing returns per tests by N6LF and others, so you will have achieved reasonable maximum performance with this setup.

  • “My vertical hears just fine.”
    Verticals are noisy receive antennas. Often, my very short beverages-on-ground have been 6 or 7 S-units quieter than the Inverted-L on 160 and allowed me to make QSOs that simply wouldn’t have been possible otherwise.

  • “The vertical should be a quarter-wave long.”
    Your Inverted-L should actually be longer than a ¼-wavelength. Making the antenna slightly longer will raise the current maximum in the vertical section well above the feedpoint (this is good). The trick, of course, is keeping the maximum beneath the horizontal portion; if the antenna becomes too long, the horizontal portion will act as a radiator instead of a capacitance hat (this is bad). If you’ve done this properly, of course, you will still need to provide some capacitance at the feedpoint. Based on modeling at my specific QTH over the years, I’ve found 135’ to 150’ lengths to be the sweet spot for 160. Again, SWR is a poor design metric — a small L-network at the base will easily solve the problem.

  • “I don’t need a feedline choke.”
    Unless your ground is outstanding (think radials over saltwater), the shield of your feedline is being used as a radial. This can cause all sorts of ugly RFI in your home and, worse, your neighbors’ homes. Consider using a commercially available choke (occasionally called an “isolator”) or construct your own.  K9YC’s popular design calls for seven turns of RG-8 through five 2.4″ o.d. #31 toroidal cores.

  • “The wire is just thrown over a branch. It works fine.”
    Verticals are easily coupled with anything nearby, including trees. While trees aren’t as bad as metallic structures, it is still best to have your vertical out in the open away from the greenery. A catenary support rope can help. Additionally, there will be substantial voltage at the end of the antenna when running high power, so be sure there is sufficient space and insulation between the endpoint and any vegetation.

  • “Feedline losses are so low on 160 that the coax doesn’t matter.”
    It’s true that loss decreases with frequency, however most coax is inherently leaky. This means that while feedline loss isn’t the primary concern on 160, intermod and mechanical considerations might be. Consider using a high quality coax like LMR-400 or hardline. This rule holds true for any antenna on any band, and especially so if you intend to operate radios on other bands at the same time. True hardline has the added benefit of direct burial and is widely available on eBay and government surplus websites.

Ten Commandments for your Inverted-L

By way of summary, here are my basic design requirements for a good Inverted-L. Many of us, myself included, can’t have all of them, but we should attempt most of them. After all, who among us is without sin?

  1. Don’t use SWR as a design metric
  2. Make the vertical section as tall as possible
  3. Use as many evenly-spaced radials as possible
  4. Use a decent choke at the feedpoint
  5. Avoid lossy bottom-loading
  6. Place the vertical element in the open, away from trees and buildings if possible
  7. Use high quality coax or hardline to feed the antenna
  8. Match at the feedpoint, only use a tuner in the shack as a last resort
  9. Use empirical performance tests; avoid “I snagged 3Y0 so it works fine” -statements
  10. Don’t use SWR as a design metric (again)

My final point is that we should never make perfect the enemy of good enough. Many of our constraints will dictate how well we can build this or any other antenna. The true test of our mettle is what we do within those constraints to maximize performance.

C U on Topband!

Mike, N1TA


Revolutionary 160m antenna at N1TA

I remember it vividly. I was standing on the edge of my toilet hanging a contest plaque, the porcelain was wet, I slipped, hit my head on the sink, and when I came to I had a revelation. A vision…a picture in my head. A picture of this! This is what makes 160m possible from a city lot: the DX capacitor! It’s taken me nearly twenty years and my entire fortune (EDITOR: $300) to realize the vision of that day. Gosh, how has it been that long?

Basic design of the DX capacitor

The capacitor is easily constructed and inserted at the feedpoint of your series-fed vertical. The smaller the vertical, the better it will work. I constructed my prototype for less than $50 with parts I had in my junk drawer and I’ve already worked 160 Meter DXCC…twice. In a single month. No listening antenna required.

The prototype in use

Fair warning: the 1.21 gigawatt power supply was very, very difficult; I recommend consulting an experienced local ham and/or astrophysicist (what difference is there?) to help you with the construction. World events and global politics may severely impact your ability to complete this element of the project, so be sure to check ahead of time.

These angry OM’s chased us home from AES!

I’ve been attempting to bring these to market, but there seems to be little interest from DX Engineering and MFJ. Luckily, Dr. E. Brown Enterprises, located in the new Twin Pines Mall next to the Hill Valley HRO, has agreed to construct and market the device to the amateur market. Please contact them directly with sales and design questions.

Dr. Brown works out of a van

The next topband season is just around the corner, so don’t wait. Get started on your own DX capacitor today and work the world before you’re outatime!


Site selection basics

The QTH hunt continues.

I have a unique advantage that I underappreciated when I was younger: I was a ham long before I owned a home (about 18 years, in fact). This means that as I shop for a QTH, I can do so with a few factors in mind specific to radio that might not be a concern for most home buyers.

But just what makes a great QTH? Well, there are a few obvious considerations – don’t be in a valley, for example. There isn’t much other written wisdom available here. A conversation I’ve long wanted to have with the biggest multi/multi station owners starts with “If you could do it again, where would you put it?

Luckily, I’ve managed to have such a conversation with Dave, K1TTT on several occasions over the years. Dave runs a helluva multi/multi station about 40 minutes from my house in way way Western Mass. Don’t live around here? Don’t worry — he’s imparted his wisdom on the world thanks to his magnum opus, Building a Superstation, a several hundred-page book available free online.

Dave explains that while he wasn’t a contester when he purchased his now famous property, asking for “good views” worked out:

The most important thing I learned was to get a Real Estate agent, but not to rely completely on the agent. I basically told the agent I wanted a house ‘out of town’, with 5 or more acres, and ‘good views’.  -Dave, K1TTT in Building a Superstation

In my case, I have a realtor who understands that I do radio stuff and I’ve more or less given him a primer on what it is I need. I modified Dave’s “good views” -trick slightly to “good views in [x] directions,” and at this point, my realtor can point towards EU, JA, and the Caribbean on command. I’ve also given some other edicts…

  • Must be in the middle of nowhere. If I can’t mow the grass while naked, the neighbors are either too close or too interested.
  • Wooded is OK, clear is best. Towers don’t like trees and this is W1; ice season never seems to end around here.
  • Minimum 10 acres. Land isn’t expensive in these parts, especially if you’re willing to move far from town and/or buy a tract without a house on it.
  • No nearby power lines. This one goes without being said.
  • Prominence trumps raw elevation.

As it turns out, willingness to buy an unimproved tract (land without an existing house) is a powerful tool. Often, buying land and building is vastly cheaper than the alternative; the danger comes in the unknowns surrounding permitting, soil, etc.. I’m willing to roll in a temporary prefab home while I build if the site is right. Not having an XYL certainly helps in this regard.

Another tactic I’ve used is consulting topographic maps and geographical information services (GIS). Most of the towns around here have online GIS systems that link to their respective assessors database. I’ve used this to construct a list of the most suitable properties in the area – even the ones not presently for sale – and set up corresponding alerts in the MLS systems. The practice has also uncovered some rare gems, like town land taken on tax title. Towns around here generally auction these off from time to time, and I now know to be on the lookout for certain properties from the auction company alert emails in the next year or so.

Timing is probably paramount with the search process. I’m in a situation where I’m not compelled to move, and won’t be in the near future. I’m in no rush to find “the most suitable site” and can concentrate on finding “the most perfect site.” That being said, if I intend to build a powerhouse station by the next solar peak, I need to get moving in the next 12 months.

If this seems like overkill, well, it is. As ARRL Contest Branch Manager a few years back, I regularly had long phone calls with operators in compromised situations, be it HOA’s, condo associations, a home at the bottom of a mountain, etc.. It is important to me that I not put myself in that situation, as I’ll certainly enjoy being active on the air for as long as I’m able and all this will [hopefully] pay off.


Station Status

One of my favorite aspects of this hobby is station design and construction. I’ve been fortunate to operate at some large W1 contest stations over the years, and I’ve absorbed as much as possible. I’ve been able to build a modest but competitive single operator station from my home QTH in Western Massachusetts, and I’m very much thankful for that. I was very active from here in 2010-2014, and even won a plaque or two along the way (most memorable was working a clean sweep and winning our section in Sweepstakes multi-op with K1NZ one year).

It’s no secret that I’m looking for a new QTH; something with lots of land and some elevation — but more on that in a later post. I’ve been so preoccupied with the hunt (and of course my professional life to support it) that my current station has sat mostly in limbo for the past three years. Here’s the overview as it stands today:

2x FT-1000MP with YCCC SO2R box
Quazi-homebrew band decoders
Homebrew 811A amplifier (more on this guy)
Homebrew 1000Z amplifier
Clipperton-L (seriously)

160: 1/4-wave sloper EU
80: shunt-fed tower; 1/4-wave sloper EU
40: Delta loop at 65′; 1/4-wave sloper EU
20: 4ele tribander at 70′
15: 4/4ele stack at 70/30′
10: 4ele tribander at 70′
RX: BOG switchable EU/SW
VHF: huh? Oh, I think there’s a vertical somewhere

Not bad for a suburban lot! The 15m system is probably the most successful of my projects. The beams are mounted on a winch-operated telescoping tower. The winch, brake, and emergency flap can be worked from inside, allowing real-time adjustment of the stack spacing. The top antenna is rotatable; the bottom is fixed at EU. Generally, both are aimed at EU and fully extended, but we’ve found tightening the spacing helps around grayline into Asia.

4/4 15m adjustable stack; top beam now 4ele tribander

While all of the antennas are still up and functional, the 80m shunt feed is presenting a pretty high SWR and the 40m delta loop looks worse for the wear but seems OK. Everything needs work, as most stations do, but things have been on hold as I’ve looked for a new place to start developing.

I’ll be detailing the process of finding, planning, and constructing a real station over the next year (hopefully) on this blog. There is a lot of information out there about constructing contest stations, but very few resources include the procurement phase. I hope to shed some light on this process as it unfurls.