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Author Topic: Where to properly connect vhf antenna analyzer  (Read 488 times)
VE4AOD
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Posts: 23




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« on: April 08, 2010, 09:43:12 PM »

Hi.Assuming you have a 5 ele vhf beam at the end of 100 feet of LMR 400, is it reasonable to expect an accurate reading/graphing of swr if you cnx an antenna analyzer (ie AEA or MFJ) to the opposite end of the cable? The low power output (+5dbm) would seem to me
to be dissipated in the long cable. Thanks for your advice....Jack
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WX7G
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Posts: 5945




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« Reply #1 on: April 08, 2010, 10:02:04 PM »

The loss of 100 feet of LMR400 at 146 MHz is 1.5 dB. A return loss measurement will be in error by 3 dB. For example a 10 dB RL (Return Loss) load will measure 13 dB at the end of the 100' cable.

You can take the measured VSWR, convert to RL, add 3 dB, and convert back to VSWR to see what the VSWR is at the antenna.

So yes, you can measure VSWR at 2 meter thru 100' of LMR400.
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VK1OD
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Posts: 1697




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« Reply #2 on: April 08, 2010, 10:06:57 PM »

Hi.Assuming you have a 5 ele vhf beam at the end of 100 feet of LMR 400, is it reasonable to expect an accurate reading/graphing of swr if you cnx an antenna analyzer (ie AEA or MFJ) to the opposite end of the cable? The low power output (+5dbm) would seem to me
to be dissipated in the long cable. Thanks for your advice....Jack

Jack,

The VSWR in this case will decrease smoothly from the load end to the source as accounted for by line loss.

In your case the line loss is 1.5dB at 144Mhz.

The VSWR calculator will give you a good estimate to the VSWR at the antenna terminals if you fill in the relevant data. So for instance, if you measured VSWR=1.50 looking into the cable with 1.5dB of loss, the calculator tells you that the load end VSWR is 1.79. If for example, there was another 0.5dB of loss between the measurement point and the radio, the calculator tells you the radio sees a VSWR of 1.43.

Owen
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VK2TIL
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Posts: 317




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« Reply #3 on: April 09, 2010, 12:57:11 AM »

But note carefully the (quite correct) caveat in 'OD's calculator which is designed for the case in which;

"... a transmitter which is designed for a particular load impedance, is connected to a an antenna of that impedance, over transmission line of the same characteristic impedance".

So, if the transmitter is 50-ohms, the antenna is 50-ohms and the intervening line is 50-ohms, all will be well but, if one or more of those components does not meet this specification, a more-complete analysis will be required.

Verb. sap.
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VK1OD
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Posts: 1697




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« Reply #4 on: April 09, 2010, 01:29:35 AM »

But note carefully the (quite correct) caveat in 'OD's calculator which is designed for the case in which;

"... a transmitter which is designed for a particular load impedance, is connected to a an antenna of that impedance, over transmission line of the same characteristic impedance".

So, if the transmitter is 50-ohms, the antenna is 50-ohms and the intervening line is 50-ohms, all will be well but, if one or more of those components does not meet this specification, a more-complete analysis will be required.

Verb. sap.

That scenario is the basis for some of the calcs.

The 'estimate' of VSWR assumes that line loss per unit length under mismatch is constant. That is not strictly true, but if you want to calculate more accurate results, you need more than VSWR, you need data that allows calculation of Gamma, the complex reflection coefficient (rather than just the magnitude of it which is the basis of VSWR). Nevertheless, the calculation of VSWR at another point is accurate enough for the OM's purposes.

Owen
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WB6BYU
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Posts: 13112




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« Reply #5 on: April 09, 2010, 08:35:46 AM »

The SWR measured at the end of the feedline tells you what the rig is seeing.
If your concern is the rig operating into a bad SWR, that is sufficient.

The actual SWR at the antenna (presuming that all the components are
designed for 50 ohms, etc.) can be calculated based on the cable losses.
(We're on VHF, so this presumes a long enough feedline that this is a
reasonable estimate.)  If you are trying to tune the antenna for minimum
SWR, you can still do so at the end of the coax:  the indicated SWR will
be slightly lower than the actual SWR at the antenna.

But if you want to know the actual complex input impedance of the antenna,
you need to correct the reading more accurately for the cable length.
Without doing so, you can't say that the reactance is 0 ohms at the antenna
just because you measure 0 ohms at the end of the coax (unless the SWR
is exactly 1.0 : 1.)
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