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Author Topic: What bands will a 20 M dipole work on ?  (Read 5185 times)
N3EMP
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Posts: 4




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« on: August 21, 2010, 06:36:34 PM »

Plan to construct a stealth 20 M dipole.  Will use it with a tuner.  What other bands can I expect this dipole to be usable on ?

Gene
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N3OX
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« Reply #1 on: August 21, 2010, 06:53:09 PM »

If you feed it with ladder line, or you can put a remote/automatic tuner right at the feedpoint, you will get good results from 30m through 10m.    Maybe you can press it into service on 40m if you're careful with the power, though such a short antenna can be rough on your tuner...

If you use coax feed , you'll likely have significant loss due to the mismatch on every band except 20m.  If you've got no other antennas for HF, you can let 'er rip with a tuner anyway and see what happens.  Usually you'll get at least a few percent of your transmitter power going out as radio waves... usually more.  Lots of people are satisfied by systems like this... they're lossy, but they're simple.   But there can be a substantial improvement if you ditch the coax+tuner feed for something else...

When I lived in an apartment, I tried feeding a random doublet with about 15 feet of RG-6 type coax... I could make contacts on all bands, but when I finally replaced the coax feed with a remote-controlled tuner out near the antenna feedpoint, the improvement was like flipping on a lightswitch on some bands.  I was a little surprised back then at how bad just a short run of terribly mismatched coax could be...


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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KD8HMO
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« Reply #2 on: August 21, 2010, 08:49:32 PM »

Would a long wire antenna work better with a tuner than trying to use a mismatched dipole?
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WB2WIK
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« Reply #3 on: August 21, 2010, 09:58:33 PM »

Would a long wire antenna work better with a tuner than trying to use a mismatched dipole?

That will depend entirely on how long the wire is, and how good your ground system is.
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N3OX
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« Reply #4 on: August 22, 2010, 09:43:16 AM »

And how noisy your house is.  

And what kind of coax you have on your dipole and how long it is.

A 20m dipole fed on 17m with 35 feet of RG-8/X coax dissipates about 55% (3.5dB) of your power in the coax.  On 15m, you lose about 75% (6dB).  You have to add your tuner's loss to this.  If it's a good external tuner, the tuner loss will be negligible.  If it's an internal rig tuner, you're stretching it significantly past the range it's designed for and I wouldn't be surprised at mildly significant extra loss.

Neither of those losses are  going to keep you from making contacts... if you radiate 50W or 25W, you'll still make a lot of contacts. 

But it's worth looking at it in from a different perspective.  For a few dollars, you can replace the coax with open wire line.  For a hundred bucks you can replace a rig's internal tuner with a decent 100W-level manual tuner.    That will make you 3.5dB stronger on 17m.

Now, let's look at something else that would make you a bit over 3.5dB stronger.  Let's say you have a 600W Ameritron AL-811 amplifier.  You could buy an AL-572 instead, giving you 1300W.  That's about 3.3dB too... and costs more than a thousand bucks more Grin 

And longer lines can be really dramatic.  If you had 60 feet of RG-8/X feeding a 20m dipole on 15m, you'd have about 7.5dB line loss.   Replacing that feedline with open wire would nearly be the equivalent of going from 100W barefoot to the 600W AL-811.

So on one hand, sometimes it's not so bad to have what some people would consider "terrible" line loss... lots of people can make the same contacts with 10W or 20W or 50W that they can with 100W.  I once tried a little experiment where I used a tuner in the shack to load my half wave 40m vertical, instead of using the matching network that I built out at the antenna.  The coax loss made my radiated power about 10W, and I still broke a big pileup on a C57  station in Gambia... but changing the lossy feedline system is a very cheap 10dB vs. anything else I could do, which is why I wouldn't use the coax feed and shack tuner on that antenna even if I *can* break some pileups with it. Grin

73
Dan
« Last Edit: August 22, 2010, 10:10:29 AM by Dan » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB2WIK
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« Reply #5 on: August 22, 2010, 12:15:05 PM »

If you can use balanced line (twin lead, ladder line, window line) and a balanced tuner, a 20m dipole can work on 20-17-15-12-10m.  If you have a very good ground system attached to the tuner, you might even be able to short the two wires of the twin lead together and feed it as a Marconi (wire vertical, top loaded by the 20m dipole legs) on 30m and 40m.  The success of doing that depends heavily on your ground system. 

Also, there's nothing particularly sacred about the dipole being resonant in an amateur band; if you picked "20 meters" because it's only 33 feet long, as opposed to 66 feet for 40 meters -- if you have the room for somewhat longer elements, like maybe 25' long each (to make a 50' long dipole) it should work on 20m and "up" and might load up well enough of 40m that you can use it there.  But again, the route to success for such a system is using balanced line and not coax.

Everything works to some extent.  With conditions as they are lately, I'd focus primarly on 20m and 40m for making contacts (and toss in 30m also if you like CW and digital modes).

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W5LZ
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« Reply #6 on: August 26, 2010, 05:08:02 PM »

The key word in all of this is "works", and that's very variable.  What might be 'okay' for one person may not be 'okay' for another.  Just too many variables for any kind of guarantee, so try it and see what happens.  If you expect anything other than sort of 'okay' on any band except 20 meters, you're probably expecting too much.
Paul
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AA4PB
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Posts: 12980




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« Reply #7 on: August 26, 2010, 07:36:12 PM »

To provide a little explanation of why the above comments are true, the amount of SWR on the coax feed line is determined by the match (or mismatch) between the coax and the antenna impedance. On any band other than 20M the antenna impedance will be far removed from 50 Ohms so the mismatch will be great and the SWR will be high. High SWR means more loss in the coax.

The tuner, located at the radio, will convert the impedance to a nice 50 Ohms for the radio but it won't do anything about the SWR on the feed line nor the resulting high loss. The actual antenna and feed line performance are the same with or without the tuner. The tuner just provides a match for the radio and allows it to deliver full power into the system - which is then attenuated by the loss in the feed line so only a small portion of it reaches the antenna.

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N2LWE
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Posts: 104




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« Reply #8 on: August 30, 2010, 10:35:04 PM »

I have a 40 meter dipole up about thirty feet in a tree in my backyard with coax straight from the dipole to the radio in my basement and have worked all bands 160-10 meters with good reports. I use a MFJ 969 tuner.
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STAYVERTICAL
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Posts: 875




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« Reply #9 on: September 03, 2010, 04:29:33 AM »

If you feed it with ladder line, or you can put a remote/automatic tuner right at the feedpoint, you will get good results from 30m through 10m.    Maybe you can press it into service on 40m if you're careful with the power, though such a short antenna can be rough on your tuner...

If you use coax feed , you'll likely have significant loss due to the mismatch on every band except 20m.  If you've got no other antennas for HF, you can let 'er rip with a tuner anyway and see what happens.  Usually you'll get at least a few percent of your transmitter power going out as radio waves... usually more.  Lots of people are satisfied by systems like this... they're lossy, but they're simple.   But there can be a substantial improvement if you ditch the coax+tuner feed for something else...

When I lived in an apartment, I tried feeding a random doublet with about 15 feet of RG-6 type coax... I could make contacts on all bands, but when I finally replaced the coax feed with a remote-controlled tuner out near the antenna feedpoint, the improvement was like flipping on a lightswitch on some bands.  I was a little surprised back then at how bad just a short run of terribly mismatched coax could be...




This says it all - this arrangement will give the best results for whatever bands the tuner can tune.
I use a quarter wave groundplane on 20m with a remote tuner at the base and it tunes from 60m to 6m, excepting 15m.
I have also used a half wave 20m dipole with 6 feet of 450 ohm ladder line being fed by a remote ATU and it also gave good results from 30m to 6m, usually omitting 15m with my particular remote tuner.

As Dan said, I was also really surprised at how much better I got out after going to a remote atu.
The military routinely uses this arrangement as well.

73s
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