Prepping and Comms: I mentioned last week that I’d offer my 2-cents worth on a comparison between off-center-fed dipoles (OCFDs) and conventional 1/4-wave dipoles. But, in one of those senior moments that somehow ended up not happening.
Before we rectify that with a (nearly) grown-up ham radio antenna discussion, however, a word (many, actually) about emergency antennas. The kind prepper sorts (like us) keep in mind for The BIG ONE, should it ever get here.
The first core concept has to do with the two kinds of radio wave propagation you are likely to encounter in any good, Irwin Allen (Hollywood’s master of disaster director) scenario. Though neither will work inside the Poseidon gone turtle, very well. Say, there’s a film that took the joy out of cruise ships, didn’t it?
The first main thing to understand is the “right antenna” depends on the kind of radio frequencies you are trying to pick up. So toss this summary into your Faraday Shield garbage can (*metal, of course) so you will know how to use that shortwave receiver if/when time ever comes. Lots of reading time when the power goes down.
AM Radio: The built-in ferrite rod antenna in most AM receivers is adequate for most needs. For nighttime operation and to hear long distances, you might ad the longest, highest piece of wire you can find. There should also be a ground wire for this kind of lash-up. A $3 grounding clamp from Lowes and some hook-up wire is all you need. The longer the wire run, the bigger the wire should be: Under 35-feet, or so, anything works. 35-150 feet AWG #14 will work. A little sag won’t hurt. Longer? #12.
Otherwise, radio wire doesn’t have to be big – unless you are running consideration transmitting power. Perfect receiver antenna for AM at night? Clamping onto railroad tracks for your ground and using a sling (to swing a water bottle up into a tree in David v. Goliath using it like a sling) and get 50-100 feet of wire way the hell up in in the trees.
Receiving only, a simple wire down from the top is fine. Transmitting? RG-58 coax feedline works. This leads into the longer discussion of 1/2 wave dipoles, end-fed half wave antennas, the importance of grounds and tuning antennas. Few preppers will care, though ideally some kind of standing wave measuring tool in the Faraday garbage can is ideal if you’re planning to transmit.
You will be flat amazed at the number of AM and shortwave band stations you can pick up on a good quality general purpose and shortwave radio when it’s hooked up to a high and in the clear antenna. OMG – it’s like magic.
SILVER LINING OF DISASTER: I hope you’re not as into ham radio as I am: I’m the guy who has figured out that EMP could take down most of the nation’s power grid and that in turn will shut off all man-made radio noise for the most part. My first box of MRE’s will be devoted to enjoying the suddenly quiet radio spectrum. (That, friends, is one sick preppery puppy, but we make no apologies since we worked a UA0 station (Russia) with just 25 watts on the low end of the 20-meter band Saturday which was fairly noisy with the beam looking out over eastern Europe and western Russia…but I digress…)
Antenna Golden Rule: Longer and Higher for all antennas up to about 14 MHz. When you go up from there, the length isn’t as important as additional height.
The Only Formula You Need: This is so simple: 468 divided by the frequency being used will give you the half-wave length of an antenna in feet.
So, you’re driving around The Big City and see a radio station with what looks like a 250-foot high tower. Having attended the ultimate short cuts for preppers with radios, you would know that the vast majority of AM stations are using 1/4 wave vertical antennas. So, you first convert that into a half wave by doubling your estimate from 250 yo 500 feet.
Then, you take the revised estimate (the half wave) and divide it INTO 468. This means the antenna you’re estimating ought to be resonant at 0.936 Megehertz.
BUT – again since you are studying under the Master’s watchful eye, you will realize that AM radio stations in the USA are spaced every 10 KHZ. Since 1000 KHz is 1 MHz, this means that the calculation points to a station on EITHER 930 or 940 on the AM dial. Your eye may be off.
Gee, ain’t this special and fun? (If you are male and way off in your estimated length, ask a woman to help. They have a much better eye for distances…though we will leave the cheap “estimated 6-inches” jokes for a less politically correct site than Urban. We realize that estimating 6-inches has little to do with gender, anymore….)
What’s a BAND
(On the run, or otherwise.) This is a small piece of the radio spectrum. Formal definitions are tough…so think of a band as the amount of spectrum where a single antenna will cover the desired frequencies.
A bit of broadcast engineer trivia: With a sharp (narrow passband) receiver, you can actually make an intelligent guess at what kind of antenna matching system is being used by the transmitting station involved.
This is because those GREAT BIG HUGE AM antennas are often required to be operated (especially at night) as DIRECTIONAL arrays, which is done with 2 (or more) towers in total.
Here’s the deal: What made some of the legendary AM Rock Stations famous was almost as much their antennas as it was their content. You see, all else being equal, an antenna matching network (in a building at the base of the main tower, usually), can either be built at a pi-section filter or a T-section filter.
The rockers were always playing the game of “apparent loudness” back in the day, so we took great pains not only to line up the antenna arrays (just so…) but ALSO to make sure that the network of coils and capacitors comprising the antenna matching network didn’t have too high a Q. Q, you have to understand, is what determines how sharp a tuned circuit is. Higher Q circuits are great for some things (carriers) but terrible the further you get off the stations main carrier frequency.
What KJR in Seattle or KFRC in San Francisco had was marvelous tuned antenna systems. At other stations – because this matching network discussion is a hard concept to convey to a general manager of a radio station who’s busy trying to bed the receptionist, lol – the money went elsewhere, so that station just always sounded a tiny bit weaker.
Of course, this could be partly compensated for with additional equalization and more compression (which is why AM music always sounded different than FM), but the real fact is that if a carrier is on (for example) 1,000 KHz and matching rolls off sharply, then by the time you get higher range vocals and instruments, they have become slightly muddy and not so “punchy.”
Remember: On AM, when you transmit a 5 KHz (5000 cycles, a high whistle) tone (audio) what you see on a spectrum analyzer is a main signal at 1,000 (the carrier) while there’s another signal (called the lower sideband) down at 995 KHz while the other audio appears at 1005 KHz and is the upper sideband.
If you ever play with AM on the ham bands, remember that a lower Q (usually larger) antenna will sound better on AM than the same transmitter in a very peaky high Q antenna. Ah, the trivia of it all…
The AM broadcast band up to about 15 MHz (at this time in the 11-year solar cycle progression) is the region of F2 – skip/bounce – which is how low MF and HF (medium and high frequency) radio gets all over the world. Wait for night which is when the F2 layer “comes down” and bounces things like crazy on lower frequencies. This is why KGO in San Francisco could be heard up and down the West Coast.
Called the MUF (maximum useable frequency) the best bounces are higher in the daytime (around 14 MHz in winter in the cycle we’re in right now) and then at night low frequency AM can be heard all over the place…
Back to the Bands discussion then:
One antenna will work fine for most of the 80 meter band around 3.5 MHz. BUT the high end of the band up around 4 MHz could be separately optimized hence hams refer somewhat interchangeably to the 80 or 75-meter bands.
Band in meters is the full wavelength. 80 meters – converts to 264 feet. Half of that is 132 feet (a half wave length) and since a 1/2 wave dipole is two quarter waves fed in the middle, each half of an 80 meter dipole will be around 66-feet.
As you go higher in frequency, the antenna becomes shorter.
Example? Sure! AS you work up the ham bands, and let’s look at 10 -meters, 28 MHz, the antennas are getting much, much shorter: One wave length here is 33 feet and a quarter of that is? Around 8 feet – which is why those CB whip antennas were mostly around 96-inches in length.
The Higher Frequency means the Shorter the antennas. Call 2-meters 6.56 feet. Divbide by 4. Now you have a quarter wave antenna 1.54 feet times 12 inches gives us? 19.68 inches. Easy, huh?
The high bands are called the VHF and UHF bands. This is mainly line-of-sight comms.
How far is VHF or UHF signal path under normal conditions? Standing on flat ground (or on a stable boat?):
If your walkie-talkie antenna is up 25 feet, then the distance out is 6.15 miles. If there was someone 12 miles out, you MIGHT be able to contact them direct, but depends on frequencies since microwave signals tend to “droop” in the middle. This is about where the conversation devolves into a microwave engineers discussion of “shot alignment” which I was doing for the .mil in Alaska at age 19 as a defense contractor employee…where were we? Oh yes…
If you’re on a cell tower at 250 feet, the comms range is 15.8 times 1.23 which is 19.434 miles. So yes, two 250-foot high towers 40 miles apart will be at the very ragged edge of each-others line of sight. Make it 35 miles and it’d be mucho reliable. Before fiber there was a real AT&T terrestrial microwave system which is why phone calls cost oodles back when, yeah?
If you EVER get stuck and don’t have cell phone coverage? The quickest way to increase your odds of getting signal is to walk up hill. Of course, then you get lost and search and rescue doesn’t have a clue since your iPhone died. It doesn’t matter since you got eaten by a bear…but you didn’t expect the End of Life on Earth to be easy, now, did you?
The Grown-Up OCFD versus Dipole Antenna Seminar (Lite)
Lite means you shouldn’t need to plot Smith charts to follow…but here goes. The easiest antenna is a dipole. Two quarter waves fed in the middle. Easy. But a “dipole” can be fed about 1/3rd it’s length from one end. Still a half wave long overall.
A “dipole” is a hunk of wire that has two radiating portions of about equal length. Since we know the resonant frequency may be estimated as 468/frequency divided by the frequency in Mhz, a simple dipole for the bottom of the 80 meter ham band would be 468/3.6 =130 feet.
Divide this by 2 and we have 65 feet either side of the center of the antenna. Because I’m lazy, I will call it 66 feet and toss it into Roy Lewallen’s dandy EZNEC antenna modeling platform which every radio junkie who transmits should have for designing and optimizing antennas.
In the modeling program the antenna is assigned a height (Z axis). Here I use 40-feet. I laid this antenna out along the X axis, but the Y axis lets you model loops, spaced multi-element (beam) antennas and such…
You also set where the feed point is (the circle with a 1 in it) and the wire size which I’m calling 0.09 inches corresponding to about #12 AWG wire if my brain’s awake and functioning yet.
The first thing we run is a frequency sweep across the intended main band of operation to see what the Standing Wave Ratio (or SWR) is. And it’s not bad… Most transmitters are quite happy with a 2:1 SWR or lower, but when you get over 3 to 1, then you’re asking for issues. Solid-state gear dials back transmit power in order not to overheat, and so on. See why a second antenna, tuned higher in frequency would be nice?
Notice on the dipole plot above that the lowest SWR occurs at 3.56 MHz and is a 1.15 to 1 SWR. It would work great.
Now let’s see how the Off-Center-Fed-Dipole compares. In this super simplified example, I just move the Feedpoint from the dead-to-nuts middle to 33% from one end. The antenna still works dandy, BUT the SWR is not quite as peachy so the signal is almost immeasurably weaker. The SWR has climbed to 1.5 to 1 at its minimum point.
Now let’s view the models and see what the wireframe view of the antenna pattern looks like. Big balloon shaped thing. Most of the radiation pattern is perpendicular to the X -axis (which was figure 1 in this diatribe). That’s what turns it “eggly” out the Y axis. A kind of broadside push. Simple dipoles are broadside firing. This broadside tendency becomes more apparent as you move the whole antenna up from the ground. Once you get 1/2 a wavelength above ground, which is 130 feet which even Ure doesn’t have the money for a tower that high…. it is nevertheless almost magical. You become one of the legendary BIG GUNS on the band.
The good news is that on the 20 meter band (call it 70 feet up), magic happens there a 1/2-wave up, too. And that is the tower height we (nearly) have at Uretopia.
So Why Mess with an OCFD?
Fine question, yes sir, yes mam, or yes [other]: The reason is that the OCFD is inherently multiband capable.
We will only go to the next higher (harmonically) related ham band to illustrate this point.
Our conventional Dipole antenna seems to look OK. But it has a few of what Elaine would call troublems.
The first of which is there is almost NO radiation right straight up along the Z-axis. Since right up (and right back down again) is the basis of NVIS (near vertical incident skywave) propagation, daytime use on 40 meters, the conventional dipole would be (how you say?) sucky.
Our off-center-fed-dipole, on the other hand is consistently consistent! Still plenty of broadside energy but more important, there’s a hoop-dee of NVIS component.
To break it down, there may be signal paths where the conventional dipole operated on its second harmonic might be better, but on average the OCFD would be more effective on (even) harmonically related bands. Fed with the right feedline, a 7 MHz dipole will give a good account of itself on the 15-meter/ 21 MHz band.
There is no free lunch with antennas (or democrats): Pipers are there to be paid. What changes is the inherent impedance of these antennas.
This comes into focus when we look the 200 ohm SWR plot of the Dipole (top SWR plot and sucking mightily) compared with the bottom SWR plot of the OCFD antenna:
Since transmitters generally like 50 ohm feed impedances, the OCFD and a matching device for 200 ohms looks like this on 40 meters:
This can be further optimized by using a slightly longer antenna (67-feet like the formula said…) and that would push the low point of the OCFD down to middle to lower end of the 40-meter (7 MHz) band.
The OCFD is a compromise antenna, though, because of a human factor: OCFD’s like to be fed with a 4:1 current balun stuck on the end of 50 ohm coax cable. 4 times 50 gets you to 200 ohms. (A Balun is a balanced to unbalanced feedline device) and it likes to have an RF choke at the antenna to feedline point to keep common mode components off the feedline.
The electronic nomenclature for “common mode components” is “noisy sh*t”.
As a practical matter, No ham I know would climb up 40 feet and install (or uninstall_) a Balun (or matching unit) just to change operating bands. The Balun may not be perfect on 80 meters, but close enough…
Because people who use OCFD antennas generally leave the 4:1 balun in the line on the lowest band all the time – THAT is one reason the regular dipole is still the winner on the lowest operated band. Going up? OCFD’s are fine.
Well, EXCEPT the way I I tried to beat that was by putting up a HUGE antenna – all 746 feet of OCFD at the 40 foot level and this gives me YUGE antenna gain toward Europe and the south Pacific from here on one particular ham band (80 meters) though I do use a 4:1 Balun.
There is marvelous gain (on the order of 8.58 dbi compared to a dipole in the 5.5 dbi range) so call it 3 db – which is a doubling of received or transmitted power. Almost as good as two 5/8th wave lengths back-to-back fed in the middle which is called a Double Extended Zepp (DEZ) antenna. The folks over at West Mountain Radio have a DEZ calculator online here.
Just remember, for low band radio antennas, it’s a design call between all kinds of factors: Gain comes at the expense of directivity: You might have a pattern that leave Japan out of the equation. Height above ground changes feed point impedance. 1/4 wave verticals – even two of them – can give you a cardioid antenna pattern which is useful. But the gotcha with verticals is that over-the-horizon noise tends to be vertically polarized, which is why a beam (or cubical quad) is a better choice for weak signals work. Unless you put in A MASSSIVE copper ground system under a vertical and bump it up to a 5/8ths wave…OMG this discussion goes on all week…
Antennas – like sailboats and politicians – are always a design compromise. If use an OCFD I liked the reviews (and price) of MacconUSA OCF 3K80 so much I bought it along with their RF line isolator as well. Well pleased.
The other company that does an outstanding job of the OCFD is Buckmaster which has a whole page related to their versions of the OCFD over here.
BUT, if you are looking for a mono band solution, then the four-legged “horse fence antenna” is a must-try, as well. These give outstanding performance on a single band. The KF4BWG four-legged horse fence antenna page is here. I’m saving up for one of those… The reason that both cage dipoles and horse tape antennas work so well is they have a lot of capture area. Single conductors are not as broad-banded.
The less expensive (2 elements) Horse Fence gets great reviews on eHam.net here. And you can waste several weekends just going to the eHam antenna ratings, clicking the reviews (5.0 is best) and then modeling them to see which is really the best in terms of price per dbi (decibels, isotropic) in your modeling….
At some point, I should stop writing on this…
Ah…how’s that…something nice to contemplate as the week begins…
PS How to Spot a Phony: There will be some hams who will insist on pronouncing the word balun as BAY-lun. These are idiots. The word as explained, derives from balanced-unbalanced which makes it BALun. Unless they are willing to make up a BAY-lunced antenna feed, discount their advice by 50 percent (or more). I use a 99% discount on such.
I learned, though, not to correct such people. It’s very useful to listen to people because over time, they all expose how much they know – or don’t – about the topic at hand.
Write when you get rich,