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Author Topic: 80M Trap Design  (Read 2547 times)
G8UBJ
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« on: May 13, 2008, 03:42:18 AM »

I run an inverted L for 40 & 80M and built an 80M coaxial trap with RG58 so that I could extend it to Top Band.
The antenna worked quite well on top band but I found it was a bit lossy on 80M. I could go to RG213 cable and larger diameter former to improve the trap but it has to be suspended so I don't want it to be to large. Is there  alternative design out there which would give good performance with less weight, Coil and capacitor?
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WA3SKN
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« Reply #1 on: May 13, 2008, 04:18:28 AM »

"Top Loading" is usually the lightest, lowest loss arrangement, followed by "traps", then inductors.
Try a "top hat" as a light weight solution!
Don't forget "linear loading" as a possible solution too!
73s.

-Mike.
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G8UBJ
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« Reply #2 on: May 13, 2008, 05:55:43 AM »

Okay, I have to space to extend the top section for 160 won't top loading de-tune the antenna on 80M?
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W5FYI
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« Reply #3 on: May 13, 2008, 06:08:23 AM »

A simple, light-weight trap could consist of a 100 pF capacitor in parallel with a 16 ┬ÁH inductor (26 turns, 2" in diameter, 8 t.p.i.), but because the 80-meter band is so wide you may want to adjust turns to make it work where you intend to operate most.
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W8JI
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« Reply #4 on: May 13, 2008, 06:21:23 AM »

I seriously doubt the trap has losses significant enough that you can measure, let alone tell by gut feelings. My actual measurements of traps like that showed loss was less than 1dB.

http://www.w8ji.com/traps.htm

What is it that makes you think the trap is lossy?? Maybe something else is going on?

73 Tom

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N3OX
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« Reply #5 on: May 13, 2008, 06:27:06 AM »

"Is there alternative design out there which would give good performance with less weight, Coil and capacitor?"

Yes, that would give good performance with less weight, but look at Tom's page before you go switching it out.

Good coil and capacitor traps seem to take losses from "measurable but maybe inconsequential" to "laughably negligible" in this service ... ;-)

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

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #6 on: May 13, 2008, 06:30:07 AM »

Actually for other readers of this thread that think "trapped antennas are lossy" it's worth going to W8JI's page and reading up.  

This is important information when trying to decide among antenna types.

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

Monkey/silicon cyborg, beeping at rocks since 1995.
G8UBJ
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« Reply #7 on: May 13, 2008, 06:53:48 AM »

"What is it that makes you think the trap is lossy?"
At the low end @3.58Mhz the VSWR was rising after a few minutes, was this due to loss and heating of the coax?
I tried a couple of different tuners but it did the same for each. It stopped as soon as I removed the coax trap but that left me with no 160M
I tend to use SSB up at 3.78 Mhz and RTTY down at 3.58 so I centred the trap at 3.67 .. I only run continuous power at 3.58Mhz should I try the same trap design with a different frequency ? Their cheap enough to knock up.

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W8JI
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« Reply #8 on: May 13, 2008, 06:11:54 PM »

<<At the low end @3.58Mhz the VSWR was rising after a few minutes, was this due to loss and heating of the coax? >>

Just because a cable or capacitor drifts, it does not indicate loss.

You might have moisture in the cable or something heating or wet between the turns. Remember even a few watts of heat, inconsequential for power loss, can change temperature enough for drifting.

You have a worse case situation when the trap is exactly resonantat the operating frequency. It is most critical for stability and has the highest loss. Moving the trap slightly above the operating range would really tame things down. You would have to readjust lengths on each side of the trap however.

If you change to a LC circuit with a ceramic capacitor and air inductor you can run into the same exact thing.

I'd suggest you change to a good Teflon coaxial cable with a Teflon or thinner non-vinyl jacket. Can you get some?? Like some RG-303 or RG-400 or a Plenum type cable??

73 Tom





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W8JI
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« Reply #9 on: May 13, 2008, 06:16:11 PM »

By the way, in the USA to "knock something up" means to get something pregnant, something that 80% of us now wish a certain political figure's father would never have done.  

:-)

Tom
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G8UBJ
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« Reply #10 on: May 14, 2008, 01:24:42 AM »

Thanks all and apologies for the unfortunate use of the term which in the UK means to build quickly, of course it has a double meaning which most comedy writers have exploited...
I will see if I can obtain some teflon coax as suggested as that will be more stable. I have been offered a high voltage 100pf capacitor so I will make up a more traditional design and compare. I operate RTTY on 3.58 which is where I experience the problem so would 3.75 be a better frequency for the trap. The band goes to 3.8 in the UK so above this is out of band but is that too far from 3.58?

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VK1OD
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« Reply #11 on: May 14, 2008, 06:02:04 PM »


You seem to assume that the trap should be resonant on the operating frequency.

Though popular explanations of the trap as an ON/OFF switches and that the 'cut off' section of conductor doesn't exist are appealing because of their simplicity, they don't work as simply as that.

There are good reasons to locate the trap resonance away from an operating frequency, losses and voltage rating are two of them.

Owen
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G8UBJ
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« Reply #12 on: May 15, 2008, 04:58:15 AM »

Having read around its clear that tuning a trap to the operating frequency isn't a good idea.
I assume that if a trap is not at the operating frequency it will be less effective in its stated purpose, so of course it will change the parameters of the antenna.
So how far above or below working frequency should it be tuned to f0 +5%? f0 10%? f0 -10%?
Once I have established that I can build the trap and start to experiment with element lengths.
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VK1OD
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« Reply #13 on: May 15, 2008, 02:16:33 PM »


What is the "stated purpose"?

Be wary of a narrow definition. For example, if the stated purpose is to be resonant at 3.58MHz, you have already limited your options. As you now know, it doesn't have to be resonant at the frequency of operation.

If you ditch the idea that it must be a ON/OFF switch (and it never is), you can define a more general "stated purpose".

For example, if you were designing a two band trapped dipole for coax feed, you might decide that the purpose of the trap to facilitate a low VSWR feed point impedance on the bands of interest.

This frees you to vary trap components and wire lengths to achieve that objective, considering efficiency and feed point impedance.

Owen
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G8UBJ
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« Reply #14 on: May 16, 2008, 03:50:19 AM »

I have an 80M inverted L vertical which I wish to extend to Top band 1.8 - 2Mhz
I will run 160 Watts RTTY (FSK) on top band at 1.84 Mhz.

I will be running 250 Watts RTTY (FSK) at 3.58 Mhz +- 10Kc

I wish to obtain a stable SWR when running above powers at both the above frequencies.

Anything else is a bonus...  

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