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Author Topic: i have a 3 ele 6m beam on atu ainly used for 50 mhz has naybody ever used on on  (Read 3783 times)
M0NST
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« on: December 22, 2012, 07:18:00 AM »

i have a 6m beam which will tune at 14MHz through atu has anybody ever used one this way and what results did they get ?
reason i have as i dont have enough space to full size beam or the planning permission other than that i would have to rely on my half size g5

I was just wondering if anybody else had tried this( the atu is ldg 200pro which will go upto 6m)
Andy
« Last Edit: December 22, 2012, 07:23:31 AM by M0NST » Logged
W4OP
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« Reply #1 on: December 22, 2012, 07:28:22 AM »

It certainly will not perform as a beam- the parasites will be invisible. My guess is that the VSWR will be so high that the vast majority of your power will be lost in the coaxial cable.

Dale W4OP
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M0NST
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« Reply #2 on: December 22, 2012, 07:31:45 AM »

the atu is taking the vswr down to 1 to1 and i have checked it with the mfj269pro it seems a little deaf  compared to the wire  but it works
(i did put 5 turns on the coax to act as choke which shoudl limit the parasitics something).
its either that or I'm buildiing  a fan dipole in the spring  Smiley
andy
« Last Edit: December 22, 2012, 07:36:38 AM by M0NST » Logged
AA4PB
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« Reply #3 on: December 22, 2012, 08:03:48 AM »

The tuner, located near the transmitter, is doing NOTHING for the SWR on the coax feedline between the tuner and the antenna (this is where the loss occurs) or the mismatch between the antenna and the feedline. The tuner is ONLY lowering the SWR on the short jumper cable between the tuner and the transmitter.

While the antenna system (including the tuner) will present a decent load for the transmitter, the antenna system will be very inefficient and the beam will certainly loose all of its directivity and gain.
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WB6BYU
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« Reply #4 on: December 22, 2012, 08:10:57 AM »

The first question always should be, what is the SWR without the tuner?

Many tuners can match a length of coax that is shorted at the end.  So just
because a tuner can match it doesn't mean it will be very effective - the power
is just being dissipated as heat in the coax instead of being radiated.

So let's try an example.  I have a model of a 6m yagi here and the impedance
on 20m is roughly 3 - j1500 ohms.  We can put that into VK1OD's transmission
line loss calculator here:  

http://vk1od.net/calc/tl/tllc.php

along with the feedline information and see how much power reaches the antenna.
If you have 15m of RG-213 coax then the loss in the coax is almost 28dB, so with
100W of output power you are getting less than 200 milliwatts radiated.  The
losses on receive will be similar, which may explain why signals seem "down" somewhat.

Now, in practice it could be better or worse than this, depending on the details of
the antenna.  A beta or gamma match on the beam will have more of a mismatch
on 20m, while if one side of the element is grounded the the boom might become
part of the antenna.  And, if the balun isn't effective at 20m, the outside of the
coax may contribute a lot of radiation also.


But the general rule is, just because you can match the coax with a tuner, doesn't
mean that the power is reaching the other end and being radiated.  It's the SWR
on the coax between the tuner and the antenna that determines the losses, which
is not affected by the tuner settings.

A simple dipole will work much better.
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KB1GMX
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« Reply #5 on: December 22, 2012, 08:51:34 AM »

simple you have a 112 inch (about that) elements in a doublet configuration as the reflector and director are largely inert.  Now a 20M dipole is about 400 inches (rounded down) making your 6M beam barely 1/4 size with about 1/16 the gain of a dipole (or about -9db) and a swr at the antenna in the much greater than 10:1 range most all of it capacitive reactance.

Add to this the coax loss at the high reflected power that is a contributor, though not as horrific as
many would think.  The rest of the loss will be in the tuner.

Does it radiate, yes, very poorly.  The choke at the antenna limits its performance as having the coax
radiate helps in this case!

You would find most anything even a simple garden dipole (inverted V, bent, Z shaped) at 20M would out perform that by a huge margin.


Allison

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KA1MDA
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« Reply #6 on: December 22, 2012, 11:51:24 AM »

According to Yagimax, at 14.1 Mhz a 3 element 50 Mhz yagi will have an impedance of about 3 ohms - j1250. The gain is 1.7 dBi (negative referenced to a dipole) and the pattern is pretty much like a dipole, with a front to back ratio of less than 1 db.
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K4SAV
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« Reply #7 on: December 22, 2012, 02:36:38 PM »

This could be a case similar to my 2 element 40 meter Yagi on 160 meters.  The SWR on 160 is about 2.5 to 1.  It hears pretty well on 160 too.  I don't transmit on it.  That can be very confusing until you consider what is going on.  The balun on this antenna isn't very high impedance on 160.  There was no reason for the manufacturer to make it high impedance on 160.  Look at all the antennas that use the feedline as a radiator.  The things they all have in common is that the real part of the feedpoint impedance is very low, the reactance is very high, they are fed with coax, and they have no common mode choke.  That pretty much describes my XM240 on 160 too.  What I have is a vertical radiating feedline with some top loading.

Note: There have been several cases of people burning out the balun on an XM240 when shunt feeding a tower at high power.

Jerry, K4SAV
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VE3FMC
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« Reply #8 on: December 22, 2012, 07:08:11 PM »

the atu is taking the vswr down to 1 to1 and i have checked it with the mfj269pro it seems a little deaf  compared to the wire  but it works
(i did put 5 turns on the coax to act as choke which shoudl limit the parasitics something).
its either that or I'm buildiing  a fan dipole in the spring  Smiley
andy

Andy you better start building that fan dipole now because you are wasting your time loading that 6 meter beam on 20 meters. I have the same tuner here, it is hooked up to my backup Icom 7000. That tuner will tune my 17 M vertical on 80 meters. But I guarantee you no one is going to copy my signal. Well maybe someone here in town!

You are losing so much power in your coax that the antenna will not radiate much RF.

Build the dipole, use the beam on the band it is designed for.

Merry Christmas....Rick VE3FMC
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W4OP
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« Reply #9 on: December 23, 2012, 10:41:33 AM »

the atu is taking the vswr down to 1 to1 and i have checked it with the mfj269pro it seems a little deaf  compared to the wire  but it works
(i did put 5 turns on the coax to act as choke which shoudl limit the parasitics something).
its either that or I'm buildiing  a fan dipole in the spring  Smiley
andy

I don't think the original poster understands that VSWR is an indication of the degree of mismatch between the antenna and the feedline and a tuner located in the shack does not alter this mismatch- or the increased loss in the cable as a result of the mismatch.

Dale W4OP
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VE3FMC
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« Reply #10 on: December 24, 2012, 10:05:55 AM »

the atu is taking the vswr down to 1 to1 and i have checked it with the mfj269pro it seems a little deaf  compared to the wire  but it works
(i did put 5 turns on the coax to act as choke which shoudl limit the parasitics something).
its either that or I'm buildiing  a fan dipole in the spring  Smiley
andy

I don't think the original poster understands that VSWR is an indication of the degree of mismatch between the antenna and the feedline and a tuner located in the shack does not alter this mismatch- or the increased loss in the cable as a result of the mismatch.

Dale W4OP

I agree Dale. Having a 1:1 SWR because a tuner is allowing the radio to see that does not equate to good power radiation from an antenna. But he will learn, although he may learn the hard way. If he starts calling CQ on 20 meters using that 6 meter beam and does not get any replies then sooner or later he will figure it all out.

That auto tuner he is using is very effective at tuning out extremely high SWR's in antenna systems. As I said I can tune my 17 meter vertical on 3.755 when using that AT-200PRO tuner. Rig sees a nice 1:1 SWR at 100 watts. Of course by time that 100 watts gets to the antenna it is likely down to about 5 watts due to the loss in the coax.
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