Getting into scanning/Listening - antenna help.

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Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Sat Sep 07, 2019 7:54 am

Thanks Chris, the bazooka/coax dipole on that site was the one i built.

Is there any harm in taping the dipole direct to a fibreglass pole or does it have to be mounted on an arm?

I don't think the fibreglass is conductive, its a telescopic kite pole ;)

Similarly for the slim-jim - can that be mounted direct to the pole?

I'm just testing stuff at present but have found the slim-jim hates being near the house walls - lots of noise created.
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Re: Getting into scanning/Listening - antenna help.

Postby m0lsx » Sat Sep 07, 2019 10:21 am

Chris P wrote: which suggested that if the angle between the two elements ie the upper and lower poles was close to 134 degrees the bandwidth was maximised and would cover a 10 to 1 range eg 50 -500 Mhz with a total length of 40cm for each arm of the V elements .


The angle of a conventional Inverted V alters the length of the legs too. Which is a nightmare if you are making an inverted V for portable use, as the SWR will then potentially alter depending upon where it suspended from & where each leg is tied too.
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Re: Getting into scanning/Listening - antenna help.

Postby Chris P » Sat Sep 07, 2019 10:27 am

fibre glass poles are fine provided they do not have a carbon fibre content . With most home brew antennas try it and see is the only way you`ll find out . It was only by doing this that I first discovered that a lot of commercially available airband/scanning antennas were worse than home brew dipoles . The main advantage of buying a commercially built antenna is the likelihood that it may be more sturdily constructed and suitable for maintenance free installation outside .Hence why I have a D777


with regards to the noise picked up from house walls ,it could be due to the use of aluminium foil coated insulation board or plasterboardand any cabling within the wall construction . in general an antenna mounted in the clear will always perform better than one close to any other object as proximity to masts, walls , trees and other antennas will detune a resonant unit .
Regards Chris aka G8FFF nipper or tazmin88
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Re: Getting into scanning/Listening - antenna help.

Postby m0lsx » Sat Sep 07, 2019 10:33 am

Dave In Herts wrote: Similarly for the slim-jim - can that be mounted direct to the pole?


Cut a very tiny piece off the tip of your pole & put it in the microwave for a few seconds. If it heats up, it is conductive. If it remains cool it's RF transparent.
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Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Sat Sep 07, 2019 10:54 am

Ok, i'm not sure i can cut any off the pole but i can ask the makers i guess.

Slightly more objective comparison...

Two aerials mounted from opposite arms of the living room centre light (its raining) so a pretty poor but stable setup. The aerials were the slim-jim and the coaxial dipole, both cut for 125MHz.

Tuned to various frequencies, looking for a steady signal - maybe not speech etc but any steady-state signal. The aerial was then swapped over and the signal strength noted.

Seems both are dead below 75MHz, the SJ is dead above 167MHz and the CDP above 175MHz.

The SJ peaks at 130MHz so maybe the 'cut' is a little off?

Both seem usable between 80MHz and 160MHz though which i think is pretty wide? A spread of +/-40Mhz from the 'cut' target.

I think this answers one of my questions - I certainly can't realistically cover 50-500 with a nest of these as it would take at least 6 to do it. 4 would do it if focused on 6m, 4m, 2m, 70cm but a 6m SJ would be 4.2m tall :)

Its all hot-air, but i found it interesting. Still not much help in choosing an aerial though :)


Frequencies - 75MHz 130MHz 140MHz 150MHz 160MHz 164MHz 170MHz 172MHz 180MHz
Slim-Jim s0 s7 s3.5 s4.5 s3 s3.5 s0 s0 s0
Coaxial Dipole s0 s3.5 s3 s5 s3 s5 s1.5 s3 s0
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Re: Getting into scanning/Listening - antenna help.

Postby radiostationx » Sat Sep 07, 2019 6:10 pm

Hi davefromhearts and well done on making the coaxial dipole from my website .

The slim Jim will be a better performer on the freq it is cut for , and in the case of a 125 or 128mhz version about 3mhz each side it's pretty good but any more and the performance drops off significantly to a point where it's useless and then slightly further out of band the performance comes back again.
In short, the jpole or slim isn't widebanded.
The coax dipole is widebanded, it doesn't have as much gain as jpole or colinear types but it gives consistently good results over the 25 or so megs you need and it gives less interference on the line due to its balanced design using unbalanced coaxial as elements.
Bowtie is similar, not a "super gainer" but a very good all rounder if you are accurate with wire bending ,cutting & make it right .
All these antennas have been around for decades and pre date scanning as we know it.

The 128mhz civil coax bazooka dipole is pretty good on mil air as well as it's not too far off a full wave in that band especially 250-280 mhz, but as you know military airband has a spectrum which is well over 100mhz wide so it's very hard to make one that will perform well ...a jack of all trades but it comes pretty close.

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Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Sat Sep 07, 2019 6:21 pm

Thanks Mike,

your reply pretty much backs my own findings with the slim-jim vs the dipole, the dipole was a lower signal but wider and more consistent.

What is your recommendation for me?? - I want to get as much of 50 - 500Mhz as i can realistically get.
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Re: Getting into scanning/Listening - antenna help.

Postby radiostationx » Sat Sep 07, 2019 10:42 pm

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.


Image

Image

Image

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.
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Re: Getting into scanning/Listening - antenna help.

Postby m0lsx » Sun Sep 08, 2019 7:12 am

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.

The speed of a radio wave in free space is 300,000 meters per second. So the simple maths is to divide 300 by the frequency in mhz. Thus it does not take a calculator to work out 50 & 500 mhz are 6 meters to 0.6 meters (60 cm.)

Yes the physical, electrical, length of the antenna will be shorter in the real world, but the wavelengths remain the same.
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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.


Image

Image

Image

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 4.2m long ???? Or maybe there are smaller styles i should look at??

Dave
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