Getting into scanning/Listening - antenna help.

A place to talk about everything Radio and Scanning

Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Sun Sep 08, 2019 7:34 am

radiostationx wrote:50-500mhz is a big ask Dave,
In physical wavelength terms ..thats about 73cms to nearly 6 metres
Having one antenna to cover all that part of the radio spectrum leads to compromises being made somewhere, more likely everywhere.
You dont mention mounting outdoors or indoors but given you have that lovely r8500, to get the best out of it outdoors is best.

One thing you may notice with the coaxial dipole and bowtie (only if you make it right) is that they are quiet noise floor antennas, they are balanced designs.
ie they are same on each half, they work either way up.
By design..Equal and opposing currents flow along their lengths, this mitigates unwanted noise from other bands/house generated electrical noise.

I have been making little half wave transformers for HF long wire (ununs ,if you like) , fully resin potted with condensation drain hole, 316 stainless fittings, silver coated ptfe wire for the windings, they perform extremely well on aeronautical HF frequencies using my SDRplay RSP1A, No ground required.. well pleased with results.
These should be good for 75-100w or so tx as well, probably more. Neat as qrp units with a 7 strands of 0.2mm ptfe stealth wire element.

I will make some a blog page to ramble on about construction and suchlike.

There isnt really a limit to what you can DIY with radio..especially antennas but 50-500mhz is sort of doable but the results I fear wont be the ones you want.
There are 2m/70cm/6m white sticks out there in "radio land" but they are not very good at any of the 3 bands..they are a compromise.
Keep on making ! The experiments are good for the mind..at times !
Read so many books, physics theory and webpages of other radio experimenters out there over the years my eyes are going square but its fun to get in there and "just do" :smiles:

Unwanted noise is your biggest enemy with a 50-500mhz requirement.
Our houses and residential areas are full of electrical noise, its getting worse and much harder to find the source.
The world is going forwards with computing and led lighting, phones, tellys ,routers the lot..we have to sometimes go back to trusted balanced antenna designs to try to combat the unwanted chuff spewing out all over the place and give us at least half a chance of receiving something we want to hear.


Thanks again, nicely made little unun as well, i did try a 9:1 balun on my long wire but very little difference, I have since ordered an MLA30 loop as i cant afford the wellbrook loop yet and reviews seemed positive for the money ;)

I am ok to mount an external pole to get just above the roof line, but preferably not with a giant array on rotators on it ;) I am also receive only. I now realise some of the issues going from 6m to 70cm, I was blinded by the trade guff about certain types covering 25MHz to 3GHz etc :smiles: I have no issues about a 3 or 4 way antenna switch so maybe need to look at ways to mount 4 dipoles centred at 6m, 4m, 2m and 70cm on one pole. The 6m dipole is still a serious item though at some 2.8m long ???

Dave
M7DJK
Dave In Herts
 
Posts: 46
Joined: Fri Aug 30, 2019 8:44 am
Location: East Hertfordshire, UK

Re: Getting into scanning/Listening - antenna help.

Postby m0lsx » Sun Sep 08, 2019 8:16 am

Don't forget that for FM, the convention is vertically polarised, so your dipoles need to be vertically mounted & not horizontally, as there is a 3db loss due to cross polarisation.

On skywave signals polarisation is not an issue, but on groundwave, it is.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via http://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 6241
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.

Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Sun Sep 08, 2019 8:55 am

m0lsx wrote:Don't forget that for FM, the convention is vertically polarised, so your dipoles need to be vertically mounted & not horizontally, as there is a 3db loss due to cross polarisation.

On skywave signals polarisation is not an issue, but on groundwave, it is.


Thanks for that, yes i would be vertically mounting. This is all VHF/UHF and apart from air-band its mostly FM up there I think ??
M7DJK
Dave In Herts
 
Posts: 46
Joined: Fri Aug 30, 2019 8:44 am
Location: East Hertfordshire, UK

Re: Getting into scanning/Listening - antenna help.

Postby radiostationx » Sun Sep 08, 2019 9:36 am

m0lsx wrote:
radiostationx wrote:50-500mhz is a big ask Dave,
In physical wavelength terms ..thats about 73cms to nearly 6 metres

I would check your maths again.. 50 mhz is the amateur radio 6 meter band & 433mhz is the amateur 70cm band & the higher frequencies get, the smaller in wavelength they get.



Sorry my mistake , 50mhz is 5.7mtrs (6mtrs or so) and 500mhz is 57cms . A large slice of the spectrum.



Dave, The unun transformers shown are 64:1 ratio and wound on a manganese-zinc powder core.
User avatar
radiostationx
 
Posts: 147
Joined: Sat Jul 14, 2018 7:29 pm

Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Sun Sep 08, 2019 9:45 am

Thanks.

Is a coax-dipole array the way forwards do you think?

I was thinking to centre 4 dipoles on the main bands and see how it works or is it best to go for a couple of bands plus civil air?

If we can do it in 4 I can use something like this switch which seems pretty suitable...

https://www.radioworld.co.uk/coax_switch/cx-sw4pl_40way_coax_switch_with_so-239_sockets
M7DJK
Dave In Herts
 
Posts: 46
Joined: Fri Aug 30, 2019 8:44 am
Location: East Hertfordshire, UK

Re: Getting into scanning/Listening - antenna help.

Postby m0lsx » Sun Sep 08, 2019 11:26 am

radiostationx wrote: Sorry my mistake , 50mhz is 5.7mtrs (6mtrs or so) and 500mhz is 57cms . A large slice of the spectrum.


No. You are ignoring the science. A radio wave travels at 300,000 meters per second. So you divide the speed of 300,000 by the required frequency in KHz or more simply 300 by the frequency in MHz. The size of the antenna is defined, in part, by the wavelength, but it is NOT the wavelength.

And the reason we use the correct mathematical length for antenna wavelengths is that they are uniform. While the physical length of an antenna is not. Outside factors can effect the physical length of an antenna. For example with an inverted V, the inner angle of the V can effect the length. The materials used will also effect the length, depending upon their velocity factors. Some coax has a velocity factor written on it for a VERY good reason. But the wavelength of the frequency remains the same & if we start by building an antenna to the correct mathematics, then we always end up with an antenna that we can tune, as it is much easier to take away metal, than it is to add it.

Wavelengths are very basic learning in radio & something you need to get right, especially if you are trying to help people. As something that is so fundamental to the physics of radio, is the foundations of later understanding. So get something this basic wrong & you are building someone's knowledge on foundations which will cause them to not understand later issues correctly.
Buy a database from Kimmy JS19 via http://ukscanningdirectory.co.uk/
Or do Google search of this forum via http://www.google.com/cse/home?cx=partner-pub-6291336405621919:2662881632
73 De Alan (M0LSX.)
http://www.qrz.com/db/M0LSX"
User avatar
m0lsx
 
Posts: 6241
Joined: Wed Nov 21, 2012 9:14 pm
Location: Norwich. TG21.

Re: Getting into scanning/Listening - antenna help.

Postby radiostationx » Sun Sep 08, 2019 11:42 am

Sorry but I did not mention antennas,the hardware or the medium the waves are travelling through, the speed or how that speed is affected by hardware or transfer medium, just the huge slice of the spectrum Dave was intending to receive.

Dave understands this and now he is considering a 4 way antenna switch with 4 matched dipoles or similar.

A jpole made from 300ohm window line the default online calc vf values of 0.96 will not work well due to the fact the electrons are slowed down because the ladder line has copper coated steel conductors and not 98+ % copper like plumbing pipe so a value of 0.88-0.9 is likely to yield much better reception when using 300ohm ladder to construct it. Some listeners make airband versions and end up with poor results because they accidentally made the antenna using 0.96vf and end up with something more suitable for 144mhz than 128mhz.



You are right on one thing, we are trying to help a person.
We are amateurs at this, even the professionals such as Rockwell international with unlimited budgets l have problems with radio reception , even they don't get things right 100%of the time as they are up against physics some of which is not entirely understood.

Recently constructed a patch antenna for Inmarsat aviation transmissions using jaero software @ around 1.5ghz. Up here the science really comes in to play and the measurements have to be very accurate. Right and left hand polarisation too. Happy with the results though.
User avatar
radiostationx
 
Posts: 147
Joined: Sat Jul 14, 2018 7:29 pm

Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Fri Sep 20, 2019 7:35 am

Ok, I'm back - had a malfunction with my account for a week (don't change your email address :shocker: ) and then a short break :smiles:

I have got a pile of bits here to start making aerials, just waiting on a mast to arrive.

Will start with three dipoles on 125Mhz/air band, 4m and 2m bands to see how it works out.

Now have a magnetic loop aerial for HF stuff as well.
M7DJK
Dave In Herts
 
Posts: 46
Joined: Fri Aug 30, 2019 8:44 am
Location: East Hertfordshire, UK

Re: Getting into scanning/Listening - antenna help.

Postby Chris P » Fri Sep 20, 2019 8:19 am

don't forget magnetic loops don't need to be mounted at height only a metre or so above ground level is fine and as far away from buildings as possible
Regards Chris aka G8FFF nipper or tazmin88
Chris P
 
Posts: 535
Joined: Sun Nov 02, 2014 12:20 pm
Location: 6.5nM west of 06 threshold marham

Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Sat Sep 21, 2019 5:07 pm

Chris P wrote:don't forget magnetic loops don't need to be mounted at height only a metre or so above ground level is fine and as far away from buildings as possible



Hmm, with our little yard i'll have to go up to get away from the house.

I have a pole going up with possibly 4 dipoles on it, i was going to mount the loop on the end of the pole.
M7DJK
Dave In Herts
 
Posts: 46
Joined: Fri Aug 30, 2019 8:44 am
Location: East Hertfordshire, UK

PreviousNext

Return to Radio and Scanning Chat

Who is online

Users browsing this forum: No registered users and 1 guest