homebrew (1600-1700 KHz) antenna analyzer

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kc8gpd
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homebrew (1600-1700 KHz) antenna analyzer

Post by kc8gpd » Sat Oct 08, 2016 8:33 pm

as everyone here probably already knows, i play with FCC legal low power broadcasting (if you can call it that at these very low power levels) under 15.219.

well i am looking for schematics for a low power (milliWatt range TPO) antenna analyzer.

i need it to simultaneously measure impedance and VSWR.

basically i am looking to build an X-Band specific low power (mW Range) MFJ type antenna analyzer

these i know are not commercially available and will need to be custom built.

either one with it's own signal generator or one meant to be driven by a low power 50 ohm transmitter into an antenna system.

i am dealing with 100mW to final RF stage (probably about 60mW TPO) transmitter levels

i want to experiment with different 3m (top load, center load, EH, Isotron, capacity hats, etc) antenna designs and see what performs best for different installation scenarios.

i'm sure there is a basic design out there i can modify for my purposes.

thanks

Robert In Denver
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by Deep Thought » Sun Oct 09, 2016 1:23 pm

What you appear to be looking for is an impedance bridge, but that will not tell you how the antenna will perform as a radiator. Neither will a proper network analyzer.

The Delta OIB circuits are published in the manuals for the various bridges and are relatively simple affairs. Here is one for the OIB-1 (this is the updated version with the meter amplifier) which doesn't have the actual schematic, and another for the OIB-2 (2-30 MHz version) with full docs.

You could also find a GR 1606 bridge cheaper than you can build something, and it will probably do fine at the power levels you are fiddling with. It is a much better piece of equipment and will read R and X, from which you can calculate VSWR.
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by kc8gpd » Sun Oct 09, 2016 5:28 pm

thanks for the info Deep thought

what kind of price should i be looking at for the GR1606?
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by Deep Thought » Sun Oct 09, 2016 9:42 pm

I've seen them go anywhere from $50 to $500. There is one on fleabay right now for under $150 shipped.

You'll also need a detector with a meter, which can be a radio with an external antenna input.
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by Dale H. Cook » Mon Oct 10, 2016 7:55 am

Deep Thought wrote:You could also find a GR 1606 bridge cheaper than you can build something, and it will probably do fine at the power levels you are fiddling with.
Bill DeFelice is interested in AM Part 15 power levels - I wonder what equipment he has used for this purpose. Since the 1606-A is specified for an oscillator voltage range of 1V to 10V it will do fine for this work.
Last edited by Dale H. Cook on Tue Oct 11, 2016 7:24 am, edited 1 time in total.
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by Dale H. Cook » Mon Oct 10, 2016 7:57 am

Deep Thought wrote:You'll also need a detector with a meter, which can be a radio with an external antenna input.
Note that it should be a well-shielded receiver to avoid contaminating the null with energy received directly from the antenna under test.
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by kkiddkkidd » Mon Oct 10, 2016 12:22 pm

There are at least a half-dozen of the Chinese VNA's on ebay whose coverage starts at or below 1mhz. Most of the MFJ analyzers start at 1mhz as well. However, some of these won't give you the sign of X.

I have recently dragged both of my old 1606's out and found that the bearings or something mechanical on the dials has become intermittent and makes the null jump around so bad that both are almost unusable. Several years ago, I talked to Dan Churchill about a similar problem with the one that I had rebuilt and I recall him saying that is a pretty common problem and the parts to fix it are almost unobtainable.

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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by kc8gpd » Mon Oct 10, 2016 12:47 pm

i'm looking at the following design which utilizes a o-scope for readings. looks easy to use and cheap to build.

http://www.part15.us/forum/part15-forum ... nna-tuning
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by Deep Thought » Mon Oct 10, 2016 4:19 pm

I am not sure why they are so obsessed with "resonance" which is merely the j0 reactance crossing point. You can easily tune +j or -j out with the appropriate series component. The transmitter still needs to see whatever impedance its output network is designed to operate into for the antenna to be useful.

A Part 15-compliant antenna is NEVER going to be an efficient radiator...it just can't be at 10 feet long. Even at 1700 KHz it is a mere 6.2° and the only hope you have is to get it to radiate a small bit of the RF energy being fed to it instead of dissipating it all as heat. The illusion of "resonance" looks good on test equipment but the antenna doesn't care...broadcast antennas are generally not resonant because it is not necessary. We use networks at the antenna tower to match the transmission line characteristic impedance in order to minimize losses in the transmission line, but it has no effect on the antenna itself. Very short loaded whip antennas appear to be 50 ohms (or whatever) when tuned to a particular frequency but that is only because of the parallel resonance between the capacitive reactance of the whip and the inductive reactance of the coil. The radiation resistance is still very, very low which makes them horribly inefficient. The project you linked to is attempting to observe the results without doing anything to measure the actual X and R of the antenna, which is essential to maximizing power transfer. As many hams have discovered the hard way, just because the transmitter is showing low VSWR it doesn't mean the antenna is tuned properly.
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by kc8gpd » Mon Oct 10, 2016 5:43 pm

for me this whole thing is a learning experience. i am by no means an antenna expert, but i learn by hands on doing then i do from text books. all this is about teaching myself broadcast engineering.

my personal part 15 doesn't go very far at all on am or fm. maybe about 200-300ft to a really good all band portable and i am using fcc certified pre built transmitters so there is not much learning experience there.

i can pick the fm up out in the garage on a table radio sporting one of those table top external antennas (with static) about 150ft away.

the am picks up nicely around the apartment and the apartment building but doesn't radiate even to a good portable much past the building.

i gave up long ago using part 15 for serious broadcasting now it is just about learning how radio works :-)

maybe if we ever get a home (and get out of this apartment) in a neighborhood with above ground wiring i will give neutral load carrier current another go.

i have also been toying with feeding part 15's (radiating cable system) placed in small nursing homes using barix boxes and running some form of companion radio for the old folks but that is far down the line.

right now i am just trying to learn, currently about how antenna's work.

once i start experimenting i will have lots of questions likely as i am long time into radio and i am a competent engineer and can fix stuff and scratch build small projects but i am by no means an expert. right now i am trying to build / buy the appropriate tools for the job.

i am also a licensed ham and i am trying for my general to get HF privileges and will start experimenting there as well once i obtain a higher class license.

i need to refer back to my CIE books as to what X is, i know R is resistance

i also snagged one of those Isotron 200B antennas for AM (less than 3m high) and that is the current antenna i will be experimenting with using a talking sign transmitter which is 100mW input and sports a output that can load into 50 ohms in addition to it's 3m wire antenna

i will be testing it ground mounted over a ground radial system and see what it will do.

i want to get as good a power transfer and maximum possible radiation from it to see how it will do.

i have no real high expectations. it is simply a learning experiment and won't be for 24/7 microcasting.

i spend a lot of time at home (permanently disabled) and i get board and when i have the energy to i like experimenting with QRP and different designs.

i appreciate the vast knowledge available here and hope you will all be able to bear the newbie questions on antenna systems.
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by Deep Thought » Mon Oct 10, 2016 7:26 pm

I guess my point is (was?) that you need to start with a realistic goal here. It's fun to build things but if they don't tell anything useful there isn't much to be learned. It is much more helpful to start with reliable test gear than to be guessing about what you are looking at. You don't have to spend much to do that. It also helps to know what the adjustments are actually doing before you try to adjust them.

You originally asked for advice on a tool to measure impedance. I gave you my recommendation and now that this is going in another direction I'll not be contributing further. Good luck.
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by kc8gpd » Mon Oct 10, 2016 7:45 pm

yea i have severe ADD and it is easy for me to go off in multiple directions.
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Re: homebrew (1600-1700 KHz) antenna analyzer

Post by Dale H. Cook » Tue Oct 11, 2016 7:23 am

kkiddkkidd wrote:I have recently dragged both of my old 1606's out and found that the bearings or something mechanical on the dials has become intermittent and makes the null jump around so bad that both are almost unusable.
One of the fellows on the GenRad mailing list on Yahoo Groups has been working on the same problem on his 1606-A. The initial and final balance controls for both R and X are variable capacitors, and a common problem as these age seems to be noise generated between the rotor and the stator of caps. Various approaches to cleaning and lubing those caps have been tried, and I am waiting to see how this latest effort turns out. Getting at the caps can be challenging, especially the X initial balance, which is inside three concentric shields.
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