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UK spotter or spotted only.

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Date:24/11/17 Time:00:04
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UKHASnet
Detecting a Pulsar at 1420MHz?

Recently Charles G4GUO had mentioned about being able to detect one of the strongest pulsars with my dish, this idea came from BBC's Stargazing Live in Jan 2016 with their focus on finding new pulsars, see https://www.zooniverse.org/projects/zooniverse/pulsar-hunters/home
As usual i got interested in that, dropped the other projects and started reading up on detecting pulsars!
Looking at the information from other amateurs i found that in theory it should be possible for me to detect the strongest one we can see from the northern hemisphere.

A few other Amateur radio operators who do EME have detected a pulsar with their systems but all with bigger dishes than i have.
IW5BHY 4m http://www.ik5vls.it/pulsar-b0329--54.html
K5SO 8.6m http://www.k5so.com/Pulsars_2015.htm
I1NDP+IW1DTU 10m http://i1ndp.altervista.org/pulsar.html
There are a number of other amateur groups who have detected various pulsars.

The best place to find out about detecting pulsars is here: http://neutronstar.joataman.net/

A pulsar is a very dense spinning star, these are formed from a big version of our Sun dying, sometimes they become neutron stars other times they become black holes. These spinning neutron stars emit signals at most frequencies across the spectrum, the higher the frequency the weaker the signal generally is but there are optimum frequencies to listen depending on the pulsar.

I have been finding out about pulsars and how to identify them, for Amateur astronomers actually finding a new pulsar will be extremely unlikely because they are such weak signals. Most use the folding technique to integrate the signal of a known pulsar, this is done by overlapping lots of chunks of receiver data at intervals of the spin period. The more data overlapped the better the sensitivity but the signal from the pulsars does vary with scintillation so sometimes the pulsar can not be detected and other times it can be quite strong.

So.. after finding this out i decided to put some code together to do this process, i already had all the hardware to give it a try.
The receiver is my 3m dish with 1.4GHz feed feed, G4DDK VLNA, 1410MHz +/-5MHz filter and an Airspy SDR receiver capable of 10MHz bandwidth.
I read in 2x the spin period of data from the SDR at 10Ms/s, these samples are then split up into ~7ms chunks and averaged to get the magnitude for that 7ms of data, it is then stored in a 200 point array, this is repeated for the rest of the 2x spin period before grabbing some more data.
These 200 points are then passed to some python code to average and then plot it over many intervals.

So far i have not been able to detect anything on 1410MHz from PSR0329+54 using 2 hour's worth of overlapped data across 10MHz bandwidth.

There is dispersion to think about but at present this *should* not cause too much of a problem. The difference between 1405 and 1415Mhz in my data should not exceed the 7ms slots i'm looking in. These figures come from the TEMPO_calc (gui) software i found by K5SO, it calculates the pulse rate against location on earth, the time and the frequency. This is based around the cut down TEMPO software from K1JT.
You can find the cut down TEMPO and TEMPO_calc software here: http://www.k5so.com/Pulsars_2015.htm
(correction 2/5/17)

PSR0329+54 is stronger at ~400MHz than 1400MHz so the next thing i have to try is receiving on a lower frequency. This leads into many more problems, most being interference but also trying to find a clear 10MHz of spectrum.. somewhere at 410MHz or 350MHz may work.

As an initial test i have built a ring feed in the choke of the 1420MHz feed, this is tuned to 410MHz, i connected up my G4DDK 70cm VLNA but it was totally overloaded from the start, inter-modulation all over the bands!
Some local pulsing signal around 441MHz was the worst offender but Bilsdale Terrestrial TV transmitter is only 15km away and the hundreds of 380MHz data signals are extremely strong too so some filtering will definitely be required.

I don't have any really low loss filters to put before the LNA so i'm going to try make an LNA from a high power Gaasfet device and then use my 2dB loss 8MHz wide filter after it and see what results i get then.
I could also try the PGA-103 LNA which appears to handle strong signals much better, need to order a new one first though as the other is at the top of my mast being used for the tropo yagi's.


Hopefully i will have more to add soon.......!











Last page added:16/08/17 15:45
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