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


160m is cool.

“Top band,” “the Gentleman’s Band,” “MF” or whatever you call it, 160 meters is a blast. And it’s surging in popularity, much thanks to Joe Taylor and his weak signal modes. You may have opinions on those modes counting as real radio or not, but your opinion doesn’t really matter. The fact is that more operators than ever are finding 160 and most of them are doing it from city lots!

The true beauty of 160 is that the playing field is pretty level; even the most extensive arrays are still very much compromise systems. Consider for a moment that a true 1/4-wave vertical made out of tower sections is only as effective as it’s radial network. Consider for an additional moment that your wire inverted-L over a more extensive radial field might just smoke that first example.

Nick, K1NZ runs an inverted-L with a single radial, no RX antenna nor amplifier, and works EU on demand with the new FT8 mode. At my current QTH, I’ve worked at least 100 countries with a simple half sloper and a 250′ beverage-on-ground (BOG), primarily on CW. Neither of us are particularly skilled with antenna modeling and both of us face space restrictions.

There’s a lot of information online for anyone looking to get on topband, and plenty of misinformation. Here’s what I’ve learned as it applies to this QTH only (your mileage may vary):

  • Verticals crush dipoles on 160. Crush is the strongest responsible word I can find to use.
  • Elevated radials are better. I found 6-8 played nice with inverted-L type antennas here over the years.
  • Buried radials are less efficient, so you’ll have to use more. The point of diminishing returns at my QTH was 30 evenly-spaced radials slightly buried or on the ground. This agrees with the consensus among various mailing list geniuses.
  • Use an amp. Absorption is very high on 160; the extra dB’s help.
  • Nobody really understands propagation this low, and the best openings may only last a few minutes — VOAcap and similar programs are critical for the serious operator.
  • Immediately at grayline (and ONLY then), my low dipoles outperform my beverages for RX and my verticals for TX. There is no good explanation for it, but ON4UN notes a similar phenomenon.
  • Beverages are cheap; build one if you have the real estate. If you don’t, you should try a BOG. If you don’t have space for that, try a Shared Apex Loop or a K9AY loop. There’s no excuse for being an alligator!
  • Learn CW. Try the JT weak signal modes. Do something new.

See you on topband!


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!


Lower 15m beam slipped

It’s been windy here the past few days and the lower 15m beam seems to have slipped a bit when I checked antennas this afternoon. Luckily, it’s restricted by the tower face and can’t go too far so the feedline appears to be alright.

Low 4L15 spun by the wind

Why does this sort of thing happen? Because the tower leg is too thin for the existing U-bolts on the boom-to-mast plate. I should have drilled the plate for smaller U-bolts before raising the antenna and fixing it to the tower, but it was a few days before CQWW and I had run out of time. There’s little sense in repairing at this point, as this antenna will be taken down with the rest in anticipation of moving to a suitable QTH. I will probably climb up and use rope to secure it, simply to prevent pinwheeling.

This antenna (and the one up top) are homemade 15m monobanders using parts from Cushcraft 4L10’s. I needed minor extension in the elements (only a few inches each) and I extended the boom. I’ll post full details later, in case anyone else wants to try this conversion. They sit on a telescoping tower that is controllable from the shack so we can adjust spacing using a ground-mounted winch. This has been an interesting platform for experiments in stack design; at some point I’ll compile what we’ve learned.