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Author Topic: Tuners for UHF VHF?  (Read 1201 times)
GRADY
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Posts: 14




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« on: March 10, 2011, 05:00:16 PM »

Ham in waiting, studying for my test. I see that the largest majority of tuners, either manual or auto, appear to be from 1.8 to around 30 Mhz. I have seen a few that will handle 6 meter, but they about stop there. Why is this? Is it that 2M and 70Cm are that easy to tune? No market for them? It just had me wondering.

Thanks
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KF5KZX just the new guy on the block!
KB4QAA
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Posts: 2371




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« Reply #1 on: March 10, 2011, 05:47:45 PM »

You are correct tuners for v/u just are common.  The antennas are typically broadbanded and the radios don't seem to be as persnickety about the SWR.   Also, tolerances become extremely tight when you get to UHF, meaning expensive to build tuners commercially.
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WB6BYU
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Posts: 13242




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« Reply #2 on: March 10, 2011, 07:34:14 PM »

You can build a tuner for VHF/UHF, but you really don't want to use one in the shack.

The reason is due to cable losses.  To minimize cable losses and get maximum power
to your antenna, you have to keep the SWR on the cable low.  If you need a tuner
in the shack, that is because the SWR on the coax is too high.   In that case it
is better to adjust the ANTENNA to get a good match (so that the whole length of
cable operates at a low SWR) than to match it in the shack (so only the short
piece between the rig and the tuner has a low SWR.)

On HF the losses in a reasonable length of coax at, say 5 : 1 SWR aren't much to
worry about.  Let's assume you are using 100' of RG-213:  using VK1OD's handy
transmission line loss calculator (here) we can see that
the cable loss when matched at the antenna is 0.7 dB, and with the added SWR it only
increases to 1.5 dB, so 70% of your power reaches the antenna.  

Repeating the exercise on 2m, the base loss in the coax is 2.5dB and the high SWR adds
a further 2dB of loss, and drops your radiated power from 55% to only 36% of the transmitter
output.  You can avoid that loss by matching the impedance at the antenna rather than in
the shack (and by using better coax cable).  That's why serious VHF operators don't use
a tuner in the shack.

There may be occasions when one comes in handy - for example, matching 300 ohm twinlead,
but it is better to build a dedicated circuit to do that.

Generally you don't NEED a tuner, even on HF, as long as your antennas have low SWR.  Many
antennas can be used on multiple bands without one, but they do come in handy when you
are using an antenna that isn't well matched - often trying to press a single wire into service
on multiple bands.  The same applies on VHF / UHF, but you are rarely trying to match a wire
antenna on multiple bands, and the losses due to high SWR usually dictate that the matching
be accomplished by adjusting the antenna instead.


« Last Edit: March 10, 2011, 07:37:38 PM by WB6BYU » Logged
GRADY
Member

Posts: 14




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« Reply #3 on: March 10, 2011, 08:42:27 PM »

You can build a tuner for VHF/UHF, but you really don't want to use one in the shack.

The reason is due to cable losses.  To minimize cable losses and get maximum power
to your antenna, you have to keep the SWR on the cable low.  If you need a tuner
in the shack, that is because the SWR on the coax is too high.   In that case it
is better to adjust the ANTENNA to get a good match (so that the whole length of
cable operates at a low SWR) than to match it in the shack (so only the short
piece between the rig and the tuner has a low SWR.)

On HF the losses in a reasonable length of coax at, say 5 : 1 SWR aren't much to
worry about.  Let's assume you are using 100' of RG-213:  using VK1OD's handy
transmission line loss calculator (here) we can see that
the cable loss when matched at the antenna is 0.7 dB, and with the added SWR it only
increases to 1.5 dB, so 70% of your power reaches the antenna.  

Repeating the exercise on 2m, the base loss in the coax is 2.5dB and the high SWR adds
a further 2dB of loss, and drops your radiated power from 55% to only 36% of the transmitter
output.  You can avoid that loss by matching the impedance at the antenna rather than in
the shack (and by using better coax cable).  That's why serious VHF operators don't use
a tuner in the shack.

There may be occasions when one comes in handy - for example, matching 300 ohm twinlead,
but it is better to build a dedicated circuit to do that.

Generally you don't NEED a tuner, even on HF, as long as your antennas have low SWR.  Many
antennas can be used on multiple bands without one, but they do come in handy when you
are using an antenna that isn't well matched - often trying to press a single wire into service
on multiple bands.  The same applies on VHF / UHF, but you are rarely trying to match a wire
antenna on multiple bands, and the losses due to high SWR usually dictate that the matching
be accomplished by adjusting the antenna instead.




Very informative, and I bookmarked the link. Obviously now I see how much more comes into play than I realized. I really appreciate your response. Another learning moment for me.
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KF5KZX just the new guy on the block!
G3RZP
Member

Posts: 4565




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« Reply #4 on: March 11, 2011, 07:02:36 AM »

Some of the older ARRL handbooks had tuner designs for 6 and 2 in them, but this was for feeding balanced lines. An open wire feeder with small spacing is quite efficient at 70cms over a long length compared with good coax.
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K8AXW
Member

Posts: 3828




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« Reply #5 on: March 11, 2011, 08:38:03 AM »

QAA came close to the reason for not needing an antenna tuner for V/UHF.  Antennas at these frequencies are very broadbanded because of the frequency vs. the element diameter.

On HF, the frequency-element diameter ratio is very high, which means that if you build an HF antenna from say #26 wire, it will be resonant (2:1 SWR and under) for a certain frequency band width.  If you build the same antenna with #12 wire it will be more  broadbanded.

When you get to V/UHF, the frequency-element diameter ratio is smaller and the bandwidth is greater.  So, no need for a tuner.  An antenna tuner at these higher frequencies also presents a whole host new problems.

QAA is also correct when he says that the radios used at these frequencies are not a "persnickety."  This is due to the design practices and needs.  No doubt some of the more technically inclined that read these forums can expound on this a great deal more.

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W8JX
Member

Posts: 5768




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« Reply #6 on: March 11, 2011, 08:40:23 AM »

WB6BYU brings up some very good points regarding line loss from SWR at VHF and UHF. Also while I have not seen data I also suspect that tuners insertion loss can be a issue too at those frequencies being that man made noise is not a issue here generally and you work signals down to "mud" and you want to minimize line losses here.
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N3OX
Member

Posts: 8847


WWW

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« Reply #7 on: March 11, 2011, 09:05:30 AM »

QAA came close to the reason for not needing an antenna tuner for V/UHF.  Antennas at these frequencies are very broadbanded because of the frequency vs. the element diameter.

It's not just the diameter effect.  Part of it is the percentage bandwidth of the bands themselves.. the bandwidth divided by the center frequency of the band.  That's what antenna bandwidth is really about, not the absolute bandwidth in kHz.

160m is 200kHz wide, 200kHz/1900kHz = 0.11, 11% bandwidth.

80m is 500kHz wide... but that's 500kHz /3750kHz = 0.13, or 13% percentage bandwidth.

2m is a huge 4MHz wide.  But that's 4MHz / 146MHz = 0.027, just under 3% bandwidth.  That's similar percentage bandwidth to 20m, which is easy to cover under 2:1 with a dipole.

70cm, 420-450MHz at THIRTY megahertz wide?  30MHz / 435MHz = 0.07, 7% bandwidth

So not only are the antennas broader because of their dimensions, the VHF bands are narrow.  As far as the antenna is concerned, the wide 70cm band is narrower than 160m and 80m.

The wire issue is still important.  A 14 gauge dipole used on 160m has about 5% 2:1 VSWR bandwidth (free space, will be much wider over dirt)... a 14 gauge dipole used on 70cm is more like 9% 2:1 SWR bandwidth... A 70cm dipole of 14 gauge wire is similar bandwidth as a foot and a half diameter 160m dipole (cage dipole would be practical).

So on 160m maybe you're trying to cover an 11% wide band with a 5% or 7% wide antenna, where on 70cm you're trying to cover a 7% wide band with a 9% wide dipole... easy.


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

Monkey/silicon cyborg, beeping at rocks since 1995.
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