Wednesday, October 15, 2014

Ham Radio - Antennas

Thanks to some comments on my Monday post, I got thinking about collecting some thoughts on antennas.  I've been a ham since 1976 and an RF (radio frequency) engineer (but not an antenna designer) since 1986, and along the way have used various antennas, both full size and smaller.  I hope this is useful.  For lots more about antennas than you can imagine, the ARRL Antenna Book is the standard ham's book. 

Let's start at the beginning: why do you need to care about antennas?  We're surrounded by radios that don't seem to care what we use.  An FM radio with a whip antenna doesn't seem to care if the whip is fully extended or not, and a lot of folks will clip or twist a wire onto the end of it to try and get more distant stations.  If you're old enough, you remember putting aluminum foil flags on your rabbit ears - the TV antenna.  Our smartphones or cellphones are radios, and they don't have bulky obvious antennas (at least, not anymore). 

It's the difference between receiving and transmitting.  Transmitting is about getting power out over the air over to a receiver.   A receiver cares about getting something into the first amplifier it can amplify.  In the case of your car radio, an AM broadcast radio or that FM radio, just getting some voltage is all it needs, and almost paradoxically, they don't need power, just voltage.  Those radios are designed with antennas as voltage probes to get a small amount of voltage (millionths of a volt will do) out of that miniscule radio frequency power going by. To get the most power out of a transmitter, antennas need to have the same impedance the transmitter was designed for, and just about all transmitters are designed for 50 ohm antennas.

The problem with saying you want a 50 ohm antenna is that they're only 50 ohms at one frequency, and you probably want to transmit in many places.  The reference 50 ohm antenna is a half-wavelength long dipole, usually made of wires.  (In theory, they're closer to 75 ohms, but in our environments  and the way we usually mount them, that value comes down.)  The length of a half wave dipole in feet is 468/f with f = frequency in MHz.  If you want a 40m dipole, then it's 468/7 or around 66 feet tip to tip.  You can do that for the 40m SSB and find a shorter antenna, so the procedure is to cut it for the part of the band you want.  You can also make a shorter antenna and tune it on frequency with electrical components. 

All of which doesn't answer the question of how you'd transmit on any HF ham band with one antenna. The most direct answer is either an antenna with empirically found dimensions - I've heard good things about the "Carolina Windom" - or a simpler design with an antenna tuning unit. 

I've personally used an off-center fed dipole with a 4:1 transformer in it for years, and the tuner built-in to my HF rigs (I think all Icoms with a tuner use the same basic design) allows it to cover the entire HF spectrum and even up to 6m.  I wouldn't hesitate to recommend this approach.  My 40m OFCD is 66 feet of wire, but instead of being attached to the transmitter in the exact middle, 33' and 33', it's fed at half of 33' (16'6") from one end, so that the long side is 49'6" long.  (Don't get hung up over exact dimensions: 17 and 49 will work just as well).  The feedpoint was hung from the side of my tower at about 18' up, and each end tied off after sloping down toward the ground.  Impedances ranged from 4 ohms on 80m up to over 100 ohms on 6m; the rig was perfectly happy tuning it and running full power.  An antenna analyzer plot of impedance magnitude, angle and SWR looks like this.  Ain't nothing 50 ohms about this antenna!  I've had literally hundreds (thousands?) of contacts with this antenna, including some pretty exotic DX on 40 and 30m. 
On a road trip playing QRP portable, I made a 17m dipole (18.1 MHz), about 12 1/2 feet on a side, and with an LDG Z-11 autotuner, I was able to operate all HF bands.  Even smaller than the home OCFD for easy packing.  I'm sure if you cut a dipole for a lower frequency it would work just as well for you with an autotuner. 

The ARRL has plans for a parallel set of dipoles which works well on the "old" ham bands, 80, 40, 20, 15 and 10, due to their harmonic relationships.  A set of wires cut for 40, 20 and 10 will work on 40, 20, 15, and 10.  I ran one of those for a year while in a rental apartment, the midpoint held up by a TV antenna pole (15'? 20?) and the ends drooping off to the ground. I was shocked at how well that worked. 

Of course, there are lots of commercial options and while I've emphasized antennas I've built, you can pay someone to make the wire antennas for you.  I've bought some baluns so I'm not A/R about doing everything myself.  The point is not to over complicate things.  These simple, wire antennas will usually work very well if you get them in the clear. 


12 comments:

  1. Don't overlook the simplicity of an all-band-doublet fed with "ladder-line" and an external balanced tuner:

    http://www.hamuniverse.com/hfdoublet.html

    I demonstrated the use of a simple doublet for HF field emergency operations at Brock's spring PATCON two years ago. Photos of the setup are at the end of outline I posted after the event:

    http://ncrenegade.com/editorial/thoughts-about-emergency-communications/

    Thanks for all your good work on the Silicon Graybeard blog ... I gotta gray beard too :>)

    Hans
    in the NC woods

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  2. For portable operation I favor the BuddiPole. It's configurable is so many ways you be boggled.

    And with one of their accessories, you can turn it into a BuddiStick, and use it as a vertical, which is what I'll be using on the Iowa this Saturday.

    It's not cheap, but it's very well made, and the Special Forces guys use them.

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  3. I love me some wire antennas for fixed HF operation. Currently I have a 200-300' NVIS loop fed with window line that wraps around the house & back yard. If I can beat the weather, my old 40m inverted V & 80m-10m doublet will go back up. The doublet was always fed with window line, and I think I've fed the inverted V with both coax or window line at various times. I prefer window line, but either works. My main tuner is an MFJ Versa Tuner II, mostly because it is relatively cheap and supports balanced feedline. My dummy load is external, the place I bought it didn't have the version with the internal dummy load and I don't mind. My dummy load sits off to the side, no problem.

    I bought the 80m-10m doublet as a kit. Remarkably the kit was cheaper than buying just the feedline. I doubt that still applies, but worth a quick check.

    If you have a tower, yagis, quads, moxons, oh my!

    For park-portable, I tend to go with simple: End Fed Half Wave wire antenna with counterpoise, cheap crappie fishing pole, and an Emtech ZM-2 tuner. In the past, I have used a SuperAntennas mp-1 vertical but tuning the coil is twitchy and carrying an antenna analyzer makes the whole thing much less fun.

    drjim, my brother has both a BuddiPole and Isotron and likes both.

    The SOTA (Summit On The Air) guys work hard to reduce weight and some fair number of them use magnetic loops. For instance, see KD5KC on qrz.com.

    Antennas are a broad and deep subject, and feedline is intimately related. For someone looking for good introductory material, the ARRL Handbook has good info. ARRL also has several volumes of Wire Antenna Classics books. If you're cheap, you can just join ARRL and snag most of the articles off the online QST article archive. They have a monstrous archive freely available to members. See also
    http://www.arrl.org/radio-technology-topics
    http://www.arrl.org/antennas

    I am unable to mention antennas without mentioning L. B. Cebik, W4RNL (SK). I bought his CD and it's well worth the money.

    From Monday:
    SiG - glad you have the 703+ and enjoy the links.
    Weetabix - if you let us know a bit more about what your intention (what bands) and constraints (HOA stealth, portable, fixed), budget ... we might be able to get more specific. The FT-897D is almost a "DC to daylight" rig; just one antenna won't really cut it. Hopefully some of the above in this and other posts will get you started. Check the blog tags (labels) on this post, they'll lead you to lots of other goodies.

    73, Jim

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  4. Reflections - Walt Maxwell
    http://homepages.ipact.nl/~pa1are/tuner/reflections.pdf 1 MB, 68 pages

    No, not Maxwell of the equations.
    https://en.wikipedia.org/wiki/Maxwell%27s_equations

    This Maxwell
    http://www.qcwa.org/w2du-sk.htm

    The QST archives are very hard to read, and they are the source of the book. I just ran across this, haven't read it yet but am looking forward to.

    73, Jim

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  5. Lots of good content here! Readers who go to the comments will get a good bonus.

    I like the summary that antennas are a broad and deep topic. I could do at least a small book on this myself. Maxwell's book, "Reflections" is definitely a good one. ON4UN's "Low Band DXing" is a good book. I haven't read Cebik's book, but have read several of his articles. He seems really solid, too.

    Buddipoles are good little antennas, Cushcraft R8s are good. GAP Titans are good. As Hans said, a multiband doublet fed with 400 ohm ladder line is a solid performing antenna; the design is in any ARRL Antenna book back almost forever.

    G8RVs and Carolina Windoms are good, but beware of any wire antenna that's promoted as magically better than other wire antennas like it.

    The "Real" Maxwell's physics is just brutal and there are no magical shortcuts, only compromises.

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  6. Thanks for the post! I think my difficulty so far has been too much information, too little space (big house on a small lot), and too little time.

    I'm interested in stealth antennas more because they seem to fit my size restrictions (no HOA or anything).

    I have several books - ARRL Antenna book, and many on wire and on inconspicuous antennas.

    I think my best bet will be a camping trip with a roll of wire and an antenna analyzer.

    Gear I have at this point:

    - Yaesu FT-897D
    - RigExpert AA-170 antenna analyzer
    - MFJ 969 tuner I got used, but I think one of the inputs doesn't tune.
    - G5RV "Maple Leaf Mini" that I'm having trouble figuring out how to get up
    - Ed Fong roll up 144/440 j-pole
    - Antennacraft ST4 discone (25-1300Mhz) I got this one to play with RTL-SDR

    I've been collecting gear against the advent of time to use it.

    Right now, I think I'm interested in NVIS for fun and figuring out which bands are active enough that I can find traffic to listen to to get the flavor before I start making contacts. Nets are appealing because they expect people they don't know to chime in.

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  7. I have a not-so-old Icom in a box around here somewhere, and Kenwood dual band mobile, which is no longer in the car because it drained the battery 1 too many times.

    I really want a high-end Software Defined Radio, because that is the future. The commercial ones are a bit pricy, but I hope to have time to look at TAPR's offerings.

    As for antennas, I am not trying to set DX records. And mostly I was limited by the space on my boat. Just needed to get out on a few frequencies.

    I have considered buying one of the All-in-one verticals. Hygain, and MFJ (or whatever part of MFJ) and a few others sell them. Just not sure I trust the reviews or not.

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  8. Wheelgun,

    There are lots of user reviews at http://www.eham.net/reviews/ that are unrelated to retailers' sites.

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  9. Dr. Jim,

    Did you get just the Buddipole, or the whole shebang kit?

    The kit looks nice, but I'm not sure on a quick review that it looked like another $200 worth. I might have missed something, though. Being ready to go out of the box with no cobbling is worth something.

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  10. Here is how one person fit a G5RV to the available space:

    http://retrotechnologist.blogspot.com/2013/03/the-story-of-antenna.html

    I like how the ladder line was brought down over/around the eaves.

    Jim R

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  11. I bought the basic "kit" some years ago, and I've added it it since then.

    I now have the heavy duty tripod, the extra length mast, the shock-corded whips, and a bunch of other bits and pieces.

    I don't even want to think about my total investment, as it's probably well over $500.

    BUT....I consider it a perfect match for my K2, which also started out with the basic K2 with the SSB adaptor. I bought it at a HamCon years ago, and it, too, has been upgraded many times with the DSP, 100 Watt amp. 60 Meter/Transverter kit, and all the other mods and firmware updates to bring it up to the current revision level.

    I built the 100 Watt autocoupler for it a year ago, and with that addition, I can now either eyeball the BuddiPole length, or toss some wire in a tree, and be on the air very quickly.

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  12. This reminds me, inevitably, of the story about the two antennas that got married:



    The wedding was only so-so...

    .
    .
    .
    .

    ... but the reception was TERRIFIC!

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