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Author Topic: Need help analyzing HF/UHF/VHF Antennas  (Read 599 times)
KD4EBL
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Posts: 75




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« on: April 13, 2011, 08:03:59 AM »

I am getting back into Ham radio after a 12 year absence. Having fun studying for my General, have made a few contacts on my old rigs. My question is rather simple. I have several HT antennas (actually Cool, several mobile antennas with magnetic mounts, a diamond X-50, and a butternut HF antenna (never used). The Diamond was kept outside and may have gotten some water in the coax which was just cut off years ago when I had to move. I really want to know how good these antennas are and if they should work on the desired frequencies. I have a SWR meter somewhere in the basement, but am wondering in this digital age, if I should invest in an antenna analyzer to help sort this out. In addition to the antennas, I also want to make sure the old coax (and connectors) I am using is still good.

1) Should I buy an antenna analyzer or use another device to check the antennas? If so, which one? I can afford a couple of hundred dollars, but wonder if the expensive MFJ models will really help me in the long run.

2) What would be a good resource (book, internet site, article, etc) that would walk me through what steps to perform?
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VA7CPC
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Posts: 2393




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« Reply #1 on: April 14, 2011, 12:14:56 PM »

For the work you want to do -- finding out whether there's a short or open circuit, and whether the matching and tuning networks in the antennas are working -- an antenna analyzer is the logical tool.  Every ham club has somebody who has one, and lends it to other members.

The MFJ-259B (and other models in that line, with extended frequency ranges and more features) seems to be the "default" inexpensive unit.  See the "Antenna Analyzers" reviews for comments, and alternatives.

It's on my "wish list".

                                Charles
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KQ6Q
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Posts: 988




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« Reply #2 on: April 14, 2011, 12:43:26 PM »

For less than the price of the antenna analyzer, you could buy new antennas and new coax. Think about it...
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KD4EBL
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Posts: 75




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« Reply #3 on: April 14, 2011, 12:49:28 PM »

Really, Really, Really, good point!
For less than the price of the antenna analyzer, you could buy new antennas and new coax. Think about it...
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KE3WD
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Posts: 5689




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« Reply #4 on: April 14, 2011, 04:07:35 PM »

Also consider a lower cost SWR/PWR meter, or perhaps two as may be needed for covering the bands of interest. 


73
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STAYVERTICAL
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Posts: 875




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« Reply #5 on: April 15, 2011, 06:34:45 AM »

Welcome back to ham radio.

My dimes worth is from your situation but advanced about 5 years in the future.
If money is a problem, then I would either buy a really cheap SWR meter, since they all work ok on HF, even the cheap CB types.
Many modern rigs also have SWR meters/indicators built in and so you could use this to trim and adjust an antenna, if you use low power and take care.

However, if you can afford an antenna analyser, it will revolutionise the way you build antennas!
This device is really the equivalent of a digital multimeter, but for RF frequencies.
By using one of these, you can immediately see the resistive and reactive components of the antenna impedance and take appropriate action.
If you have even the most basic antenna analyser, as long as it shows resistive and reactive impedance elements, you can use it to calculate coax velocity factor, coax loss, capacitance and inductance, and many other useful things.
This is because from the impedance elements you can use simple mathematical formulae to calculate all these other things.
More expensive meters have these calculations coded in, but its really simple anyway, and only needs basic multiplication and division, nothing esoteric.

As regards meters, I have both a VK5JST kit which I built (around $140 Australian from memory), and an MFJ269.
On HF (which is all the VK5JST meter is designed for), they both read the same, but the MFJ269 also does up to 2 metres for impedance.
On 70cm the MFJ269 only gives SWR, not impedance, but this is useful nevertheless.

So, if money is not a real object, I would recommend getting an antenna analyser, it will take the mystery out of what is happening with your antenna and feedline, and is useful for everything from antennas, feedlines (is the loss increasing due to water etc), measuring inductance and capacitance, velocity factor, setting up traps on antennas, measuring loading coil Q, useful as an RF signal generator and can even be used as a grid dip meter with a simple dongle on its output terminals.

One caveat, the MFJ269 has a known quirk, where if you turn on power with the UHF switch pressed in, it can blow something inside the device. So, if you go with the MFJ, be very very aware of this - it is written up many times in the manual, but is very easy to forget .


Good luck and again, welcome back.


« Last Edit: April 15, 2011, 06:41:41 AM by STAYVERTICAL » Logged
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