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Author Topic: Open Wire or Coax  (Read 1254 times)
PENDING
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« on: January 03, 2001, 12:06:49 AM »

I don't have my license yet.  But I'm sure I will very soon.  I read a lot about "open wire" antenna feeds but I can't figure out how to attach it to one of those coax connectors (SO239?) and all of the radios I have looked at only have those coax type connectors.  Can I connect open wire "twin lead" to a coax connection?  If not, how would I connect it to the transmitter?  If I use a short piece of coax from the transmitter to a box with a pair of binding posts can I connect the twin lead to the coax that way and if so, do I need one of those baluns to match the coax and twin lead?  
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K0RS
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« Reply #1 on: January 03, 2001, 05:57:32 AM »

Hi Pending.  The short answer to how you connect a balanced parallel line to an unbalanced SO-239 connector on your rig is that you don't.  At least not without an intervening device.  That device, in the simplest form, would be a balun.  The term "balun" is short for "balanced to unbalanced."  A balun will typically have an SO-239 at the input and 2 lugs for the balanced line at the output and will transform the impedance of the 50 ohm rig output to match the transmission line.  So not only is the balun a mechanical device to allow convenient connection between dissimilar transmission lines, it also has electrical properties...namely the transformation of impedance in a given ratio depending on its internal construction.  Any ratio is theoretically possible.  Common ones are 1:1, 4:1, or 9:1.  There is only one problem with this simplistic scenario: the impedance ratio is only valid for one frequency.  Despite the fact that the parallel line will have a charcteristic impedance (probably 450 or 600 ohms) the actual impedance presented to the balun will vary widely from several ohms to perhaps several thousand ohms when an antenna is connected to the far end and the frequency is changed.  This is where an antenna tuning unit or ATU comes in.  Most ATU's have a couple of variable capacitors and a variable inductor, as well as a balun, inside. This allows the operator to adjust the impedance of the antenna circuit to a value that makes the radio happy.  With modern radios this is invariably 50 ohms.  The ATU will have a coaxial input to match your radio and standoff insulators to connect the parallel transmission line.  Wallah!  You're now in business on many bands and frequencies.
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PENDING
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« Reply #2 on: January 03, 2001, 05:29:15 PM »

Thank you.  I have read a lot of specifications on transmitters and transceivers.  I don't remember reading any that were balanced outputs.  They all seem to use coax connections.   I plan to use a dipole (balanced  ..  right?)  so I guess that I'll either have to build a balun or find an ATU when I set up my station.  I would rather build a balun than buy one because that will help me to know how they work inside.  The way you explained it helped me a lot.  Thank you.

Amy
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« Reply #3 on: January 03, 2001, 05:59:25 PM »

I have enough room in our yard to put up about 150 feet of wire.  A Long Wire antenna is unbalanced  -  therefore I could run a single shielded wire from the center pin of a coax connector to the end of a Long Wire.  As long as the transceiver was grounded I wouldn't have to worry about a "ground reference" for the antenna.  Does that sound like a better idea than a dipole?

Amy
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WG7X
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« Reply #4 on: January 03, 2001, 08:56:34 PM »

Amy,

Hi. I think that you would find that the long wire to the coax connection would not work very well either, and that is because the RF will seek a ground, and that would cause the equipment to become "hot" with RF.

Most antennas of that type (long wires) need a counterpoise to work against, and the counterpoise is normally at least a quarter wave length of wire for the lowest frequency in use.

If you intend to work more than one band, may I suggest a simple dipole, fed with ladder line through a tuner? This is simple and works well. The first respondent covered that pretty well.

If you intend to try single-band operation at first, a simple resonant dipole, fed with coax, will work amazingly well, and needs no additional equipment.

Personally, I would reccomend that you try a single band with a coax fed dipole for your first effort. Things are alot easier that way. Then, with a little experience under your belt, you can go for a multi-band antenna.

Good luck, and let us know how it works out, OK?

Gary, WG7X
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AC5UP
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« Reply #5 on: January 04, 2001, 12:32:56 AM »

Amy:

You may want to re-think the longwire idea after a look at what I published in the October 2000 edition of the Oklahoma DX Association Newsletter: (http://www.qsl.net/okdxa)

--------------------

Here at Casa QRM, the weather switched from ‘Broil’ to ‘Simmer’ last month and I did a little antenna work that was rewarded by an abundance of ‘first call’ contacts during the Worked All Europe contest. I was hot into Eastern Europe and probably relied on good band conditions more than an over-achieving wire at the far end of the coax, but I’m not complaining.

Last spring I got a wild hair and took down the 20 Meter Extended Double Zepp to try out a non-resonant terminated long-wire. Bad Move. Although I had a dead flat SWR on 40 Meters and above, the performance on 20 and up was dismal with spurs from two local AM stations popping up throughout the HF range. I know multi-band antennas tend to be a compromise, but this was ridiculous. Maybe the EDZ had me spoiled, as it was one of the best mono-band wires I’ve used.

I was tempted to try a non-resonant Folded Terminated Dipole but, after reading the modeling comments by L. B. Cebik, W4RNL about mediocre TX performance, the hassle of building a 16:1 balun and weatherproofing the 800W non-inductive resistor didn’t appeal to my inner sloth. As a general rule, any antenna that requires a complicated matching device is probably more trouble than you need. I opted for an old favorite, the Off-Center Fed Dipole or, as it’s often called, a “Coaxial Windom”.

I put one up a few years ago that worked well, but the rig at that time had no Auto-Tuner. On some bands the SWR power fold-back circuit was working overtime and I wondered if I could do better. Never could tweak it into complete sweetness. But, have you noticed how the second try usually turns out better than the first? Same here. In its simplest form, the 136’ OCF Dipole doesn’t work on 30 or 15 Meters, and mine needs a little help from a tuner on 75 Meter phone from 3.75 to 3.99 MHz, but the SWR is almost flat on 40, 20, 17, 12 (all full band) and 10 Meters (28.2 through 29.2 MHz). Plus, it loads nicely on 50.125 MHz and that’s a good assortment. I didn’t expect much fire in the wire on Six, as the balun is working above its design limit, but a little testing on a local Net tells me it radiates ‘Not Bad’.

As for performance, Saturday night on 20 Meters during the WAE contest saw Lithuania, Croatia, Russia, Slovenia and England in the log while Sunday morning added Bulgaria, Latvia, Germany, Poland Lithuania, Slovenia and Belgium on 10 Meters. All barefoot at 100 Watts, all on the first call. I could have worked many more, but I had things to do and wasn’t in a contest mood. All I wanted was a little antenna check, and the QSO on 12 Meters with OM5DP in the Slovak Republic indicated the OCFD does haul the freight.

If you want an OCF Dipole in your yard, you’ll need a pair of supports in the 30’ or better range at least 100’ apart. Running the wire on a true North-South line with the short wire at the South end works well for me. I used a pair of 30’ push-up TV masts (easy to find at your local Handy-Guy mega-store) topped with 5’ of gray 1.5” PVC conduit to give 3’ of separation between the wire and mast. Glue a “T” fitting on the top as a wire guide, then cut four slits in the bottom and hose clamp it to the mast. You’ll need 140’ of 12 or 10 gauge THNN wire, solid preferred, two toroid cores of Mix 61 or 43 material, plus the usual rope and hardware. In my case, I bought some mid-sized Mix 43 (850mu) toroids on eBay last year. Although 125mu is the textbook choice for HF, the 850mu cores have worked well with fewer turns.

You can make a career out of comparing all the variations of the OCF Dipole and, as the old joke says, six web pages will give you the eight best ways to build one. Here’s mine: First, you build a two-core 4:1 Guanella ‘current’ balun. Don’t use a Ruthroff ‘voltage’ balun, stacked cores on beaded coaxial lines, or a single-core Guanella with this antenna. The dual-core (or dual rod) 4:1 Guanella is as close to ‘transparent’ as any balun I’ve used and I tried plenty of different baluns when I was tinkering with the terminated long wire. Trust me on this. You’ll find a ferrite rod design (same wiring as a toroid balun) at www2.dynamite.com.au/vk1brh/CM4Balun.htm. Mine are wound with 18ga stranded silver-teflon on beefy little 1” OD toroids about 3/4" long that hang an inch or two below the insulator. Drop the RG-8X coax feed straight down to the ground and I used RG-213 from there to the shack. This makes a break point for maintenance, mowing and the ground rod I’ll install whenever I get around to it. Waterproof the barrel splice with Scotch 33 or 88 PVC tape. (don’t waste your money on cheap electrical tape as you may regret it after the first good rain) For the ‘Magic Wire Lengths’, I found 45’ 6” on the short side and 138’ overall tip-to-tip is a good start. Prune the long side (only) in 3” steps until the bands drop in where you want them. If you’re a CW guy, you probably won’t cut anything. And, in case you’re wondering how you squeeze 138’ of wire between supports 100’ apart, the ends droop. The masts here are 120’ apart with the wire tips coming down at a 25 degree angle.

You can learn more about the OCF Dipole at http://www.cebik.com/groundup.html as well as other antennas worth considering. It’s an excellent series of articles and well worth your time, although some of the charts for the 80 Meter OCF Dipole are a bit contradictory and some of the text needs a little editing. Another good source for links to antenna projects can be found at http://www.dxzone.com. Be sure to visit www2.arrl.org/tis/info/pdf/9008028.pdf to see how a pair of shorter wires can be added for 30 and 15 Meters. I’ll be doing that to mine, but it’s been working so well I’m not inclined to ‘fix’ it just yet. You’ll also find a variation on the feed method at http://users.erols.com/k3mt/windom/windom.htm, but I’m a bit leery about the balun design used by K3MT as I’ve had better luck with Jerry Sevick’s three-core model. Look it over, and when you need a break from antennas, http://www.hypertools.com can be amusing for the formerly OD OM.

---------------------

Hope this gives you something to consider, as two local AM broadcast stations made the longwire a poor choice at my QTH. My OCF Dipole continues to work well and the only thing that's going to take it down is bad weather.

73 de AC5UP, OKDXA Newsletter Editor.
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RobertKoernerExAE7G
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« Reply #6 on: January 04, 2001, 03:29:39 AM »

If you will be taking you tests with VEs, ask them to find someone to help you get up one (some) simple wire antennas.  I used inverted Vees at first.  They are simple to make, and are easy to use with co-ax cable.  One simple design is to have a "fan" inverted Vee that has Vees for 40, 20, 15 and 80 meters if the support is high enough.  All of these antennas are liked to a common center point and "fed" with one co-axial cable. I used an 11 meter, CB, ground plane antenna for ten meters.

The hams who give you your first tests will probably have a few antenna books you can look at, and probably show you a fan dipole or inverted Vee.

Hang in there -- you will have a lot of fun when you get your ham radio ticket.

73
Bob
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W1QWT
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« Reply #7 on: January 04, 2001, 11:04:37 AM »

A couple of comments. The impedance transformation ratio is a function of the square of the turns ratio and is independent of frequency. You are right that the impedance at the end of a transmission line varies with frequency but the transformation ratio will stay the same unless you vary the turns ratio. Therefore the transformed impedance will change with frequency and not the impedance ratio.
Also another fine point is that the ATU doesn't adjust the "impedance of the antenna circuit" it just transforms it to what the output stage of the transmitter is i.e. usually 50 ohms. It does this by conjugately matching the impedance presented to the input of the ATU i.e. canceling out reactance and transforming the resistive part. Also note that there might still be a mismatch at the dipole to transmission line junction at some frequencies. If you study Smith charts you will see how this all works out at the end of the transmission line away from the dipole. If you change the electrical length of the transmission line then the impedance will move on the polar smith chart. At least this is what I seem to remember from my Field Theory and Transmission line course from many years ago.
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WB6BYU
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« Reply #8 on: January 04, 2001, 11:55:15 AM »

Well, you sure got a lot of advice!

Here is a simple summary:  Most rigs are designed to feed coax
cable and to see a 50 ohm load.  If you want to use open wire
feeders or a long wire, you will need some sort of antenna tuner
(aka "matchbox", "ATU", "Transmatch", etc.)  For reasonable power
levels you can build your own without too much trouble, customized
to your own needs.

A simple dipole antenna fed with coax provides a reasonable match
to 50 ohm coax cable.  You can use a balun at the feedpoint to
convert between the balanced dipole and the unbalanced coax, but
usually the antenna will work just fine without one.  You can add
dipole elements for several bands to the same feedoint so you
don't have to switch cables when you change bands.

There are many other options for antennas - some with direct coax
feed, some need a tuner.  You may want to experiment with several
different ones over time.  Or you may just want to put up something
simple and spend your time operating instead.  But the first step is
to get SOMETHING up in the air.  Then you can see if it works for
your interests.

One tip:  whatever you use for antenna supports, use a pulley and
halyard (or similar).  This allows you to lower the antenna for repair
or experimentation without having to climb a tree or lower a mast.

Good luck! - Dale WB6BYU
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PENDING
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« Reply #9 on: January 04, 2001, 11:15:28 PM »

Thanks again to all of you.  I truly believe I now have a MUCH better understanding of "how to feed antennas"  and I'm going to build one now for practice.  I will need it anyway when I get my license.  I have decided to build a dipole to operate 40 meters on CW and I will try to build a transmitter because I already have a receiver.  From what you have told me that should get me started and it shouldn't cost very much.  I will keep your call signs and maybe I'll be able to contact you in the next month or two on 40 meters.

73  Amy
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