Assessing aerials for RX use

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Assessing aerials for RX use

Postby Dave In Herts » Tue Sep 24, 2019 3:31 pm

I am trying to learn how to assess the best bandwidth for various aerials, I have a NanoVNA device which is a fun little gadget so I can at least get some data but data is not much use without interpretation- thats where i need help :smiles:

This is my 146Mhz double-bazooka dipole...
Image

Same results but as LogMag S11 display...
Image

1 - It seems resonant at 140Mhz, presumably as i left the ends a little longer than calculated so trimming would lift the frequency - am I correct?
2 - is SWR a good chart to look at?
3 - This one seems ok from about 114Mhz to 190Mhz ?
4 - is there an SWR value or a logmag dB value where its safe to say "thats no good at that frequency?

Please don't forget this is for RX only, I know a bent coat-hanger will pick up but I like to figure out how and then get things better, all part of the fun :smiles:
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Re: Assessing aerials for RX use

Postby radiostationx » Tue Sep 24, 2019 7:41 pm

Is that a sweep from a coaxial dipole by any chance Dave ?
The reason I ask is ,The usable bandwidth is very wide.
Did you test the antenna in free space (ie no obstructions you were not standing by it).
The leads from the nano vna arent very long but nice little gadget and reasonably priced for hobby electronics..You seem to be getting a solid enough plot out of it and for £50 or so its not going to break the bank..I fancy one to try out.
I know the unit uses a si5351 clock chip which are very accurate and very versatile for making VFOs and signal jennys.
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Re: Assessing aerials for RX use

Postby Dave In Herts » Tue Sep 24, 2019 7:59 pm

Yep, double-bazooka dipole, RG213 element.

Not really mounted well, just low in garden but away from building and me etc.

Its a fun little gadget, just interpreting the data is unclear.
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Re: Assessing aerials for RX use

Postby Dave In Herts » Wed Sep 25, 2019 7:08 pm

So, what readings/results would indicate that an aerial is no good beyond a certain range?

I did read somewhere that readings much above -10db are poor or an SWR of 1:3 for reception trouble is i have no idea of the 'correctness' of those suggestions :smiles:
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Re: Assessing aerials for RX use

Postby radiostationx » Fri Sep 27, 2019 6:32 am

Hi Dave,
The logmag graph shows the return loss, you can see that there are 2 defined dips in the chart and this is where the antenna gives best performance. You have an excellent figure of -27db for 141mhz and -23db for 124mhz. There are 2 dips with slightly higher return loss around 131mhz and 151mhz but still the antenna is performing well here.At the same time the performance is poor at 96mhz fm which is good for you.
I cannot see a marker on the graph which on other VNAs you can hold/freeze the graph and set a marker along the graph (looks like a triangle icon) to inject a signal at that point.
If you shorten the antenna stubs the dips will move to the right along the x axis and the swr chart best swr will also be further to the right which was your intended design freq.

As you can see the coaxial dipole is more forgiving than say a j-pole where the graph will usually show only 2 dips typically, 3 if you are very lucky.. and the best swr (below 2:1)will be in a 3-5 mhz section of bandwidth.
The dipole you made is very usable for 146mhz if you trim slightly, start with only trims of 5mm or so at each end then run the sweep again to observe the shift in swr and the dips in the logmag s11 screen.
They should not change shape but merely shift the pattern along to the right in both instances.
If you observe your graphs now you can get a good idea of where the dips in the logmag should be when ideal for your design and work towards that trimming little by little.
It is likely you will get a portion of the airband with very good rx as well as 146 intended.
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Re: Assessing aerials for RX use

Postby radiostationx » Fri Sep 27, 2019 6:32 am

Hi Dave,
The logmag graph shows the return loss, you can see that there are 2 defined dips in the chart and this is where the antenna gives best performance. You have an excellent figure of -27db for 141mhz and -23db for 124mhz. There are 2 dips with slightly higher return loss around 131mhz and 151mhz but still the antenna is performing well here.At the same time the performance is poor at 96mhz fm which is good for you.
I cannot see a marker on the graph which on other VNAs you can hold/freeze the graph and set a marker along the graph (looks like a triangle icon) to inject a signal at that point. Maybe im expecting far too much for a £50 device !!
If you shorten the antenna stubs the dips will move to the right along the x axis and the swr chart best swr will also be further to the right which was your intended design freq.

As you can see the coaxial dipole is more forgiving than say a j-pole where the graph will usually show only 2 dips typically, 3 if you are very lucky.. and the best swr (below 2:1)will be in a 3-5 mhz section of bandwidth.
The dipole you made is very usable for 146mhz if you trim slightly, start with only trims of 5mm or so at each end then run the sweep again to observe the shift in swr and the dips in the logmag s11 screen.
They should not change shape but merely shift the pattern along to the right in both instances.
If you observe your graphs now you can get a good idea of where the dips in the logmag should be when ideal for your design and work towards that trimming little by little.
It is likely you will get a portion of the airband with very good rx as well as 146 intended.
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Re: Assessing aerials for RX use

Postby Dave In Herts » Fri Sep 27, 2019 7:15 am

Thanks for that.

So would you say a figure of -10db or better is an indicator or usefulness, so that one although it has a dip at 124Mhz its a bit poor at the main air band centre of 127Mhz so it will function but not as well as one centred at 127Mhz?

I do have one for 125Mhz which was the first one i made, now I have an Idea of what to look for I'll take some more readings and keep a screenshot of the results for later comparison.

These graphs were taken on the pc software for the Nano it has no markers etc but is much easier to read.
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Re: Assessing aerials for RX use

Postby radiostationx » Fri Sep 27, 2019 9:07 am

Hi Dave, easy goes with the trimming and make sure that you are accurate with the measurements and cuts at each end.
I take it you have made the double bazooka using the guide in my blog page.

http://www.merseyradar.co.uk/airband-radio/home-made-coaxial-dipole-antenna-for-civil-airband/



If you run a wider sweep say 110-400 on the 125mhz version you made first you will find that there will be another (possibly 4) dips in the military airband and the SWR will be quite good there @ around 300mhz too although the mil air band spans a very large chunk you will get good performance on a fair part of it.
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Re: Assessing aerials for RX use

Postby Dave In Herts » Fri Sep 27, 2019 11:52 am

Yep thats the one :thumbup:

I have a slim-jim made from ladder-line, I'll dig that out and measure it too, my guess that it will be pretty narrow banded as that was the reason i moved to the bazooka dipoles :)
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Re: Assessing aerials for RX use

Postby radiostationx » Fri Sep 27, 2019 12:09 pm

Some inaccuracies occur if too much of the sheath and braid have been removed from the centre , I give 20mm as a rough guide if using rg-213, for a 75ohm feedline if you are skilled at cutting /stripping you can get a better 50ohm match if you can strip out 10mm-12mm only but here there is more chance of encountering stray strands of braid shorting the antenna at the wrong point.

Generally speaking, with coax bazooka more errors in building occur at the feedpoint than at the stubs. Also builders can find high swr if moisture has found its way into the feedpoint and shorting it .

The moisture gets drawn into the gaps in the braid by capillary action...(something you will be very familiar with Dave judging by your website !) That's why it's a good idea to try to" seal up "as much as possible at this location on coax dipoles (see military version notes on my blog page).

Other faults can be caused by poor contact when soldering at joins because some builders are too shy with the heat and/ or refrain from tinning up first before applying larger amounts of solder into the join or contamination in the join..pre prepping the bare copper with solder flux paste will normally get rid of any nasties 1st time of asking and it will make the solder flow better.

Have fun Dave experimenting with your new gadget..I may put one on the Xmas list.

I will send you a PM , I have an off topic question for you.

Mike.
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