Shortwave Antennas Page 8

Copyright © 1999-2016 by Harold Melton KV5R. All Rights Reserved.

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Understanding Shortwave Antennas: Page 8

Simple Antennas

Now that we know our antennas should be resonant, let’s see how to make them resonant.

First, are we transmitting or receiving? Transmitting antennas are much more critical than receiving antennas, although a receiving antenna can transmit if it is resonant (or otherwise matched) and if it can handle the transmitter’s output power. A transmitting antenna can also receive, and will do so particularly well on the frequency for which it was designed.

If you are running a CB or HAM station, you already know something about transmitting antennas. Amateur operators, in particular, must pass exam questions on antenna design and theory.

If we are operating a shortwave radio, we want it to quit fading out in the middle of our favorite shows.

Now that you are ready to get brilliant, learn rule #1 of antenna design: All antenna designs are a mixture of compromises. Just like boat hulls and airplane wings.

What we need to do, therefore, is identify our particular need and design an antenna which is optimized to fulfill that particular characteristic - whether it be directionality, gain, or bandwidth, or just all-around good performance.

I have been an antenna experimenter for over 35 years. My goal has always been to build cheap, simple antennas that work well. It’s all in the numbers. I once had a $25 multiband shortwave antenna in the attic that performed as well as any $150 manufactured SW antenna. I had a discone scanner antenna made of brazing rods and a PL-259 that I built for about $10. I once built a 10-element 2-meter yagi from scrap TV antenna parts. Some of my other antennas are shown in articles on this site.

You can string up a wire just about anywhere and get a good signal on shortwave. Fifty feet of very fine wire, strung along the ceiling on thumbtacks, will give you much more signal than the telescopic whip that comes with portable radios. Telescopic antennas are way too short for shortwave frequencies!

There is one fundamental rule for receiving antennas: It must be at least ¼-wave long at the lowest frequency you plan to use. Thus, if your lowest regular listening is on 3315 kHz, your wire should be 70 feet long - minimum. Obviously, the 5-foot whip antenna on your shortwave is just a bit too short. For much better performance, it shoud be ½ wave long on your lowerst frequency (a Zepp). Multiwire fan dipoles are best of all.

Let’s design a decent longwire antenna for general shortwave listening. We will want to listen down to 2500 kHz (2.5 MHz), then analyze its performance.

The formula for determining the ½ wave length of wire is: 468 ÷ f (MHz) = feet.

In our example, 2.5 MHz is our lowest frequency, therefore: 468 ÷ 2.5 = 187.2 feet of wire. That’s a lot more than most city lots can handle!

Okay, let’s say we’ll design it for 5 MHz, and be willing to accept slightly reduced performance down to 2.5: 468 ÷ 5 = 93.6 feet of wire. We can handle that.

Longwires are usually strung up something like this:

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This arrangement keeps constant tension on the wire while allowing the tree to sway without breaking the wire.

Electrical suppliers carry 500-foot rolls of #14 stranded THHN wire (about $50). Electricians frequently have scraps and partial rolls. Farm supply stores carry #17 aluminum fence wire (about $15 for ¼-mile roll). Any wire will do — but some will last longer than others. Stranded, insulated wire (#14 THHN) works well.

Classic Longwire Installation

This page details the installation of a relatively safe, good-performing, long-lasting shortwave antenna. The #1 rule is: Where you scrimp on quality is where it will eventually break!

Yes, you can simply hang a wire out the window. But experience shows that a properly installed antenna that is mechanically and electrically sound, and a properly grounded radio, will consistently yield better performance and reliability. It really is worth the extra effort to use good materials, solder, and weather-proofing.

Wires should be insulated, stranded #18 - #14. A jack may be soldered on to plug into your radio. If the radio has no external antenna jack, solder an alligator clip to the antenna wire and clip it to the telescopic whip.

Which Direction Is Best?

Usually, directly away from overhead power lines.

If the lines run across the back of your property, go up the back of the house, over the roof, to a tree in the front yard. If the lines run across the front of your property, go up the front (or side) of the house, over the roof, to a tree in the back yard. You can also run a wire along eaves and/or the top rail of a wooden privacy fence.

Run the longwire as far from, and as perpendicular to, the power lines as possible. This will help reduce noise. If you have buried power lines, run your antenna any way you like. In the USA, pointing your longwire northeast will help bring in European stations in the daytime, on the higher shortwave bands.

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Continued…

37 thoughts on “Shortwave Antennas Page 8
  1. Hi,
    First of all, thanks for all the long work and dedication to helping others enjoy the radio hobby.

    Second, I made this exact longwire antenna a few years ago working from your diagrams, with the spark plug arrestor. The spark plug is just a foot or so off the ground, mounted to the side of the house. The ground rod is right underneath. For a feed wire, I connected some RG-59 coax with “Siamese” combo wire, the type used for analog security cameras that has a pair of power wires. Despite having both the shielding around the coax AND the pair of power wires running the length of the wire, I never attached either to ground. I soldered a 1/8″ jack to the end and plugged it into the antenna input of my Realistic DX-390. It seems to work pretty well.

    I just purchased an RTL-SDR unit and want to connect this feed line to it but I’d like the ground coming in to where the RTL and radio are. My plan then is to use either the shielding and/or the pair of power wires as the ground. Any preference there?

    I also read on RadioReference.com (https://forums.radioreference.com/threads/sma-connector-for-sw-longwire.364513/) That I should impedance match the line and they recommended “Balun One Nine v1 – 1:9 HF Antenna Balun and Unun with Antenna Input Protection” Upon re-reading this I see it also performs input protection.

    And, I read above that you possibly recommended an isolation transformer to shunt the feedline to ground. I’m unsure about all of that and my first search for that came up with expensive power isolation transformers. Probably not what you were referring to. My second search came up with: “MFJ Enterprises Original MFJ-915 RF Isolator 1.8-30 MHz, 1500W PEP, SO-239” I found on Amazon? about $40?

    Thanks for the help!
    I salute you: o7
    (I learned that emoji playing EveOnline. pretty clever)

    • Howdy,
      You will want to get the Balun One Nine V2, it uses a better Coilcraft transformer. Screw that onto the rtl-sdr. From the balun’s screw terminals, run twin-lead (the 300 ohm TV ribbon cable will do fine for SWL) outside. Connect one side of the ribbon to the ground wire at the spark plug, the other side to your long-wire antenna at the spark plug tip.
      I would not use the CCTV cable as a transmission line, as you don’t know its characteristics in TL mode. Just use all its conductors tied together as one wire for your long wire antenna. Or better, get a roll of #14 or #16 THHN stranded insulated wire and use that for the antenna, as it’s thinner and lighter than the CCTV cable.
      The “input protection” used with some rtl-sdrs is just a pair of diodes that conduct and short the input at greater than ~0.7 volts. That will clamp a voltage spike but won’t handle any current, so they handle little spikes but not anything serious like a nearby lightning strike. Always disconnect when hearing thunder! The V2 balun has a 2-part terminal block you can separate.
      The isolation transformer I was thinking of would be a simple 1:1 RF transformer wound on the appropriate type toroid, like a FT-140-31 or 43. Nothing like a power transformer, except the same schematic. The idea is it would not stop spikes but would drain off DC static build-up. I don’t know how well it would pass RF.
      73, –KV5R

      • Hi again!
        Thank you very much for the reply!
        I may not have described my antenna correctly. I do have a #14 THHN stranded wire for the antenna. It’s a little over 100′ I believe. Where it meets the house, it goes down the side 20′ close to the ground, to my spark plug arrestor. At that junction, the RG59 takes over and center conductor connects to the longwire.

        I had never connected the coax’s shielding braid to ground. It is siamese cable and has a pair of power wires running the length of the coax.

        I’m planning on using all three, braided shield and both power wires to be the ground. I was hoping that was ok.

        • Ah, OK, yes, that will work. Without the shield connected (at both ends), it’s just more antenna wire, not transmission line. You don’t need to use the power wires for for ground though. Just ground the shield near the spark-plug, and on the radio end put some kind of RG-59 connector (usually an F, or a PL-259 with a UG-176 adapter in it) then get an RG-316 pigtail adapter to take that to SMA for your SDR dongle.
          Hope you got the RTL-SDR.com V.3, as it’s the only one with software-switchable direct sampling mode, so you can receive below 25 MHz without any HF converter.
          As upgrades, you might consider (1) the Balun One Nine (and as before, change the coax to 300-ohm ribbon cable); (2) broadcast AM & FM filters (broadcast stations make intermod messes all over an RTL-SDR); and (3) a USB Type-A extension cable, so you can put the train of dongle & filters out of the way (and not bust your computer port).
          Please let us know how it all works out!
          73, –kv5r

    • A balun (actually, a common-mode choke) may reduce noise if used on a feed line (parallel or coax), to reduce the amount of in-house electronic noise getting on the feed line.
      For a single wire that comes all the way to the radio, as shown above, it will make no difference, since there is no feed line to balance.

  2. I want to build a SW antenna for RTL – SDR. I have an old antenna tower outside the house that is 30′ tall. The pole barn is ~150′ away. I can run a #14 THHN stranded from the top of the tower to the pole barn 14′ off the ground. There is some RG-6 running up the tower that I can route through the house attic and down to my computer.
    There are high voltage transmission lines 1/4 mile East of my house. 765KV! Wow
    Do you think I will I be able to get any signal at all if I build this antenna?

    • Yes, if the transmission line isn’t noisy.
      Check the voltage to ground for induction from the HV transmission line; those rtl’s have little or no front-end protection. You might need an isolation transformer or something to shunt the feedline to ground.
      Experiment!
      –KV5R

    • Wow, this was along time ago. I finally got around to making the antenna. It is #14 wire 161′ long running East and West. The power lines in the back field run North and South. I made Delrin insulators and brackets at each end.
      I measured the voltage to ground and I have 0.536 VAC in the line.
      Is this detrimental? What can I do to remove it?

  3. I have a back property line that’s 255 feet long, I could attach a wire from 8 foot posts for that length. Is this too,long? Should it be an exact length? If it’s 8feet off the ground is that sufficEnt for reception? The plan is to receive only, no transmit plans. To connect the ant and to my future Radio location, does the wire that links to the radio have to be connected exactly in the middle of the antenna? The antenna will be nearly due north/south.

    • Yes that should work quite well for shortwave reception. Better for domestic, not so good for foreign DX, at that height. 255′ will take you down to about 1.9 MHz at good efficiency.
      Connect feed-line to center if your radio has a 50-Ohm jack; otherwise, it’d probably be better to just bring one end of the wire all the way to the radio. Look at your radio’s manual to see if it wants coax or long-wire for external antenna.

  4. I have just purchased my first portable shortwave radio, it is a tecson PL-880. I am wanting to install a point gap lightning arrester on a random length long wire antenna, I am concerned about the grounding there does not seem to be anywhere to ground the radio how would you suggest that this be done? Thanks for your help.

    • I looked at the manual and it doesn’t say anything about grounding it. However, if the ext. ant. jack is a 2 conductor type, the “shell” side could be grounded (with the jack’s “tip” connected to your long-wire antenna).
      In any case, always disconnect the external antenna when you first hear thunder, as a point-gap arrestor will not arc until several thousand volts.
      73, –kv5r

  5. Hi Harold,
    I’ve spent many hours reading your posts and I appreciate the knowledge and effort you’ve spent blogging.

    I’m a bit confused about the “twisted coat hanger” you suggest for the antenna support on p8 of SW antennas. It seems that the antenna wire would slide right through the twists. So what is it accomplishing? Where is the strain relief?

    This just tells you what I’ve accomplished so far:
    I’ve got WSPR running and receiving stations across the country. But only after I set up an outdoor wire antenna that is 2 stories high and about 60 ft long to a tree. Used a fishing pole to cast a small weight and pulled an intermediate twine before pulling rope up through the tree. But strain relief at the near(house) end is temporary that needs to be addressed. So far I haven’t wired up any grounds or used a counterpoise. Before this antenna I tried 2 different indoor antennas that were not very successful.

    I’m using a 9:1 unun at the end of the wire right before it attaches to sdr (SDRplay RP2) after coming into the house. I’m contemplating moving the unun up to the near end support and then running coax down (~25 ft) into the house.

    Thanks,
    Pete.

    • The twisted wire strain relief is just stiff wire. You wind it around a drill bit or something about the size of your antenna wire, clamped in a vise, 10 turns or so. Then remove the bit, clamp one end of the coil in vise, grab other end with vise-grips, and stretch it way out, until it’s like a very elongated corkscrew. Then put a loop in one end for lag-bolting to house. You then wind the antenna wire into that long corkscrew and it will grip the wire over its entire length (a foot or two), thus providing strain-relief.

      If it slips, stretch out the corkscrew longer, or start over and make another one with a smaller I.D. The I.D., after stretching, needs to be much smaller than the wire for it to grip. As the wire pulls on, it further stretches it, gripping the wire tighter, sorta like a Chinese-finger toy.

      They work really well for coax (and extension cords) but I’m not so sure about small single-conductor wire. You might need to make the corkscrew out of smaller wire like maybe #17 electric fence wire, or even picture-hanger wire.

      Your SDR setup sounds nice. I run a cheapo RTL-SDR dongle here sometimes; they work pretty well if you filter out the FM broadcast band with a 30-40dB notch.

      73, –kv5r

  6. Making a shortwave antenna was a lot of fun for me as a kid! I use to tie the end of my copper wire to a heavy rock and throw the wire over a tree branch of an old oak tree! The antenna worked well everytime!

    • Same here. My first SW antenna went out the window, over the roof, to a Cottonwood tree. About 75 feet total length, of which 50′ or so was ~12 feet high. Got all the strong stations (VOA, RM, HCJB etc) on old Hallicrafters. SW was so much better in those days.

  7. I am new to Short Wave, just received a Grundig Satellit 750 for Christmas and was looking for guidance on antenna design. This is good advice for a novice. Thank you!

  8. You mention here running the antenna line along the top of a wooden fence. I presume insulators would be used to hold the line parallel to the top of the fence. But here’s my thought: What if the fence is a metal chain-link type? Could it act as a counterpoise, or would the metal just muck up reception?

  9. I have almost exactly 100′ from the 2nd floor window I want the antenna to go in, out to a pole in the back yard, So I’d like to make the 94′ longwire antenna as a reception antenna, not for transmission. At the point the antenna reaches the house at the 2nd floor, it will need to go down 20′ to the lightning arrestor near the ground rod, and back up the house 20′ and into the room. Does that 40′- 50′ length down and up help or hinder the antenna for reception? What if it *were* used for transmitting? Then what?

    • Hi,
      Good question! The 20′ down and back up would not help reception, and would pick up a lot of household electronics noise.
      I’d just put the spark-plug right outside the window, in an L-bracket, then run a ground wire down to a rod. That way, the “down” part isn’t part of the antenna until the plug arcs.
      Remember that “lightning arrestor” is a misnomer — it’s really an “impulse shunt,” grounding high-voltage impulses from nearby lightening strikes, and offering no protection from direct strikes. Also, always disconnect from radio when not using it.
      For transmitting, a 20′ down-up would complicate matching. Again, I’d just put the spark-gap at the entrance point.
      73, –kv5r

  10. Hi. Thank you for your very clear and helpful site. I am a novice swl, and wanting to put up a non-obtrusive long line. Question:How much does having the wire go around corners, (not one straight line) effect performance? Thanks.

  11. “I had a $25 multiband shortwave antenna in the attic which would outperform any commercially-made $150 antenna”.

    I’ve seen many youtube videos with similar claims. Not that I don’t believe you, but just curious, are those professional electrical engineers that work for these companies not good enough to design a good cheap antenna? You’d think that the antenna industry is highly competitive so they wouldn’t intentionally overprice their stuff too much? If you made it for $25 buying retail parts, they would probably have access to cheaper wholesale prices for mass production and could make it much cheaper and still sell for $25 at a profit, no?

    • No. There’s a lot of overhead in any small manufacturing business besides sourcing parts. Commercial facility, utilities, insurance, accounting, taxes, order processing, packaging, shipping, customer service, returns, and most of all, employees’ labor, insurance, taxes, and benefits. All that overhead, and more, comes before making any profit.
      That’s why the DIYer can make a $150-something for $25, on relatively simple things like antennas.

      • Thank you, that is a very interesting topic for me that applies to any diy project. What you said may be true for the U.S. businesses. But what about China! Many of the overhead factors may not apply to them since they just copy ready made designs and manufacture them cheaply in a sweat shop. Not sure about antennas but I did various microcontroller projects etc. and very often buying individual parts at retail prices is more expensive, since ready made products are so dirt cheap on Ebay! So it is really hard to motivate yourself. Of course there is a fun factor and that seems to be the main (only?) driving force behind most of the electronics diy projects… I am glad that this may not be the case with antennas!

        • Yeah, that’s why I said “”on simple things like antennas.” For any SMT-PCB device, the big automated factory is certainly appropriate.
          A while back I wanted a good 5V 2.5A regulator for my Raspberry Pi. I could have rounded up the parts and built a very simple (and ugly) buck regulator, but hey– $12 delivered one complete with LED readout, heatsink, and even a microcontroller and two tiny buttons for calibration! I couldn’t have built one so small, or with those features.
          Manufacturing always wins on high-volume and complex products, but SWL antennas are low-volume and simple, usually just wire and feed-line. And they are fun DIY projects!

  12. Hi! 🙂

    Regarding soldering the jack to the antenna, could you briefly tell me which parts of the jack are soldered to which parts of the electrical wire? The wire I own is 14ga insulated copper electrical wire.

    Also should I get a TS or TRS jack (2 or 3 contacts) ?

    Thanks a lot for your work.

    • Most portable SW radios will have a 1/8th-inch (3.5mm) TS plug. The antenna wire connects to the tip of the jack, the ground wire (or coax shield, if used) connects to the sleeve.

  13. Hi,
    The insulation on the wire should be enough, as long as it is unbroken where it touches wet wood. It should work fine, but even better if you can get it up 20 feet or more.
    I put one in for someone many years ago that was about 50 feet of #28 (very fine) green enameled magnet wire stapled under the eave (backside of fascia board). The antenna was almost invisible, and it worked fine for the domestic US stations like wwcr etc.
    73,
    –kv5r

  14. I have purchased a Grundig Satellit 750 shortwave radio. My first attempt at an outdoor antenna was to tie one end of 18ga insulated copper wire around a piece of wood, string the wire 60 feet, 8 feet of the ground. The other end of the wire also wraps around a piece of wood and then comes into the house and is connected to the external antenna connection on the radio. I can’t judge if the antenna is working well, since this is my first antenna.
    Question
    Wrapping the wire around wood, is this acceptable, or should the ends of the wire outside be insulated?

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