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Build Your Own Antenna - Discuss topics for building your own antenna.

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  #41 (permalink)  
Old 03-07-2018, 6:51 PM
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Aha, the screen shot helps tremendously.

Maybe I can explain this for you, I hope it goes from my head to my hands in a way you understand.

In picture #2, where you are sweeping the antenna from 132 to 173, the analyzer told you the antenna loves the SWR of 1.27:1 at 162.800MHz. The different color vertical dotted line in the center of the screen is the 142.000 MHz you were looking for. Since the SWR is flatlined at that frequency at the top (SWR 19.9), your antenna is terrible for the VHF Ham band. Basically, the antenna is more suited for NOAA Weather.

In picture #1, 151.500MHz is the vertical dotted line in the center of the display. The location of 144.500MHz is indicated by the white triangle at the bottom, with the SWR of 19.9:1 for that frequency showing at the upper right in red numbers. The other triangle at the bottom of the dip is 162.800MHz even though the frequency is not shown.

The last picture is just the readout of the final result of that antenna tested at 144.400MHz, which is not good. The only result that is fairly decent there is the Zx.

Hope that helps.
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Old 03-07-2018, 6:59 PM
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NC1-
I understand what you're saying perfectly, it is exactly what I have seen and said. According to the meter, the antenna should cause the radio to literally explode when used at 144. Except...there are two radios, two OEM antennas, and the one being tested here (they both test about the same) is a gen-you-whine Kenwood that has had a LOT of use over the years. And somehow, it has performed perfectly well on a par with dozens of others, participating in networks and long distances without any problems. It hasn't exploded, it hasn't shut down from SWR problems...it has performed as well as anything else out there.
Either handhelds have quietly been made to work at incredibly high SWRs, or there's something very wrong with this picture.
I'm almost afraid to see what this will show if I go take it out to the mobile antenna. The big dual band mobile antenna, which certainly has more than 19" in the VHF portion.
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Old 03-07-2018, 7:43 PM
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The only other thing I can think of is where you are taking those readings. I have the same analyzer and have noticed that if I scan an antenna about 18 inches from my face, it will have a different reading than if I hold it out at full arms length. It also changes if I am near the TV vs outside. I think they are prone to stray RF or nearby objects. Laying it on a table is also bad.

For a true reading, go outside, hold it at arms length, and then scan the antenna. I "fixed" a stock antenna that wanted to be on 426MHz to work on the UHF Ham band and now it really works better than ever. I was targeting it to be resonant on 446.000MHz, and do believe I did a good job on this one.
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Old 03-07-2018, 8:28 PM
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We have two threads on this, why?.
I think they need to be merged.

Here is the other one:
https://forums.radioreference.com/am...queer-vhf.html
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Old 03-08-2018, 3:07 AM
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Quote:
Originally Posted by NC1 View Post
For a true reading, go outside, hold it at arms length, and then scan the antenna.
Does anyone use their handheld radio or scanner at an armlenghts distance?
Of course not. And you should measure your antenna in the position its gonna be used.
You have no use of values from an "ideal" antenna if its not being used as such.
If the antenna behaves differently when hold in a position where it is actually used then the antenna needs to be tuned to compensate for that extra stray capacitance.

/Ubbe
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  #46 (permalink)  
Old 03-08-2018, 7:30 AM
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Quote:
Originally Posted by Ubbe View Post
Does anyone use their handheld radio or scanner at an armlenghts distance?
Of course not. And you should measure your antenna in the position its gonna be used.
You have no use of values from an "ideal" antenna if its not being used as such.
If the antenna behaves differently when hold in a position where it is actually used then the antenna needs to be tuned to compensate for that extra stray capacitance.

/Ubbe
Yes. Obviously.

But if you want a true reading of the antenna itself, then you do it under ideal conditions.

Do you think manufacturers test their antennas 18" away from objects? No, because it gives a false reading. Same thing with Base and Mobile antennas.

If you know what the antenna is capable of doing, then you can adjust your installation accordingly. A location may make your install absolutely awful when compared to what you saw when you tested it. Change the antenna location, or move some objects away, and you can get it to perform better.

That is why you test them under ideal conditions; it gives you a base line reading to work from.
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Old 03-08-2018, 7:39 AM
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Quote:
Originally Posted by NC1 View Post
Do you think manufacturers test their antennas 18" away from objects? No, because it gives a false reading. Same thing with Base and Mobile antennas.
It depends on how the manufacturer designed and tested the antenna.
I have seen some HT antennas that read good when mounted directly to the analyzer held in front of your face and others that only read good when mounted on a metal ground plane or radials.
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Old 03-08-2018, 8:20 AM
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NO - this was a major error I made when testing the antenna. Clearly, I'm sure we all agree that the object would be to test the antennas on a radio - but this is impossible to do accurately, because the introduction of the meter introduces a big problem. The VSWR and other measurements are made using the meter chassis as the ground plane, and this is going to9 be different from the radio - and a Baofeng 888 size radio will be a very different to a fully metal chassis bigger radio. I discovered this when experimenting with a 1980s Icom IC-32 dual bander and a UV5 Baofeng. Swapping the radios made a difference to the test. Holding a radio, with attached analyser, with the antenna on that really skews the results, and the shape and size means you have to hold it, because the weight of all that lot on the SMA socket is a bit worrying. So when I did one test, balanced on the bench - picking up the VSWR meter in my hand, and holding it close enough to read the display changed the VSWR.

This was why I had to devise a repeatable ground plane for my experiments. I didn't use a large, very good ground plane, but something with more surface area. I know this is not representative of how the antennas will be used - but to get meaningful comparisons - you have to have a constant test setup. The conclusions I came to in the tests were than none of these antennas is truly wonderful, and only a small number have any real life gain, and then only at very specific frequencies. When used with a typical radio, the performance will be even worse.

Antenna manufacturers test their antennas on as good a system as they can - so their gain figures are always better than real world tests - which are much less repeatable.
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Old 03-08-2018, 8:33 AM
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Quote:
Originally Posted by nd5y View Post
It depends on how the manufacturer designed and tested the antenna.
I have seen some HT antennas that read good when mounted directly to the analyzer held in front of your face and others that only read good when mounted on a metal ground plane or radials.
Measuring "good" is a very relative term, both in terms of frequency and desired result by the user.

I'm sure if you look it up, the manufacturers have their own standard operating procedure for testing the antennas, and I'll bet it will be under optimal conditions.

This is the part where adjusting your installation comes in to play if you do not get your desired outcome. You know what the antenna is capable of and it is up to you to get the most out of it.

Purchasing an antenna and installing it is just a starting point because there are always adjustments and compromises that must be made in any installation.
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Old 03-08-2018, 9:22 AM
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Quote:
Originally Posted by paulears View Post
Antenna manufacturers test their antennas on as good a system as they can - so their gain figures are always better than real world tests - which are much less repeatable.
I think that is only true for reputable manufactuers with competent engineers and probably not for the Chinese junk made for the ham/CB/scanner market.
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Old 03-08-2018, 12:32 PM
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NC1-
Merging threads- Yes, a good idea. I had started the other one before I stumbled on this one, that's why there are two threads. Splicing/merging requires a moderator. (Beetlejuice, Beetlejuice, Beetlejuice.(G)

Nearfield interference is a good thought, but even at full arm's length there's no change. And, again, comparing it with a VHF-only antenna, that doesn't change at arm's length, or at "book reading" distance, where the display can be read.

Tom-
Chinese junk...well, that might apply to the scanner but I think the Kenwood antenna was made with the Kenwood radio, in Japan.

And while Paul probably has something in the "needs ground plane" arena, again, the odd reading is only with a dual-band antenna. Both UHF and VHF single-band antennas test out correctly without any extra ground plane. Having a metal case on the hand-held meter *probably* comes close to the design situation of a handheld being used in the hand.

I'm wondering if there isn't something in the way they generate the signal, that is a kludge getting false readings from ?resonance? in the odd-fractional-wavelength of a dual band antenna?
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Old 03-08-2018, 3:27 PM
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Quote:
Originally Posted by NC1 View Post
The only other thing I can think of is where you are taking those readings. I have the same analyzer and have noticed that if I scan an antenna about 18 inches from my face, it will have a different reading than if I hold it out at full arms length. It also changes if I am near the TV vs outside. I think they are prone to stray RF or nearby objects. Laying it on a table is also bad.

For a true reading, go outside, hold it at arms length, and then scan the antenna. I "fixed" a stock antenna that wanted to be on 426MHz to work on the UHF Ham band and now it really works better than ever. I was targeting it to be resonant on 446.000MHz, and do believe I did a good job on this one.
What product are you using to measure your antennas with?
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Old 03-08-2018, 7:13 PM
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Quote:
Originally Posted by milcom_chaser View Post
What product are you using to measure your antennas with?
It's a Surecom SA-250. I checked the readings I got with my friends MFJ-266c and the results were within about 2%, so I know it is working correctly. I can't imagine not having one now.

Last edited by NC1; 03-08-2018 at 7:19 PM..
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Old 03-09-2018, 3:04 PM
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Another question.

What is the output wattage of the SA-250 Antenna analyzer?

The specifications state "RF Output Power: 1-1.5V(5 - 10dB)"

How does that translate into Wattage?
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Old 03-09-2018, 3:21 PM
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It is an analyzer, you cannot talk on it like a radio so the output power is not relevant.
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Old 03-09-2018, 3:27 PM
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Yes I know it is an analyzer and you can not talk, that is not the purpose of the dummy load here.

I want to check the accuracy of the analyzer.

it obviously sends an output signal to the antenna in order to test the antenna, I want to see the accuracy of the device in order to make corrections to the readings it gives me to more accurately determine the performance of any antenna I would test with this unit.
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Old 03-09-2018, 4:37 PM
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Yes, another question regarding the SA-250 Antenna Analyzer.

Perhaps someone with the same SA-250 might be able to take a shot at answering this question.

I noticed under the "System" settings there is an item marked "Offset %".

What would this setting be about?

Anybody?
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Old 03-09-2018, 5:38 PM
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If it puts out between 1 and 1.5V then its about 20 to 30 milliwatts.
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Quote:
Originally Posted by Mikejo View Post
Another question.

What is the output wattage of the SA-250 Antenna analyzer?

The specifications state "RF Output Power: 1-1.5V(5 - 10dB)"

How does that translate into Wattage?
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Old 03-10-2018, 9:02 AM
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On the SA-250 Antenna Analyzer, under "Settings" there is an item we can adjust called "Offset %".

Does anyone know what this "Offset %" adjustment is for?
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Old 03-10-2018, 9:39 AM
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Quote:
Originally Posted by Mikejo View Post
Does anyone know what this "Offset %" adjustment is for?
My guess would be reference frequency oscillator adustment.
Is there any documentation for that analyzer?
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