Getting into scanning/Listening - antenna help.

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Re: Getting into scanning/Listening - antenna help.

Postby m0lsx » Mon Sep 02, 2019 6:06 pm

A bow tie will work on lower VHF frequencies no problem.

The Bowtie is a variation of the hour glass antenna.

http://www.arrl.org/files/file/QST/This%20Month%20in%20QST/December2018/Stanley.pdf

I would centre a airband antenna in the centre of the spectrum. Like that it will have it's optimum spectrum across a bigger chunk of airband. Although a bowtie or hourglass antenna should be fairly wideband.

I would use the shield of the coax on the lower part of a antenna. As the core is the part that will take the signal into your radio.
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Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Mon Sep 02, 2019 7:07 pm

Ok, thanks

more food for thought :thumbup:
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Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Tue Sep 03, 2019 11:25 am

While i'm getting ready to mess with VHF/UHF aerials, my research keeps throwing up discones as being a good all-round style for RX - is this true?

Using this calculator I can match most dimensions to a commercial discone i bought many years back but was never much good, it had a load of verticals and coil wound radials etc, been in the junk pile for years :biggrin:

Image

It asks for the minimum frequency wanted but only quotes a 10:1 frequency range - if its just 10:1 how do commercial items suggest ranges of 30Mhz to 3Ghz?

If made for a base of 50Mhz a 10:1 would get me up to 500Mhz so cover all of the VHF bands, nothing listed between 30Mhz and 50Mhz so no need to go there?

It would be fairly large but do-able.

Any thoughts?
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Re: Getting into scanning/Listening - antenna help.

Postby m0lsx » Wed Sep 04, 2019 10:24 am

Discones like the one above are excellent on VHF/UHF because they are wideband antennas.

The ones I linked to earlier. The one ChrisP made is a variation on the discone & equally good on broadband & having more metal in the air, probably better for weaker signals too.
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Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Thu Sep 05, 2019 8:00 am

m0lsx wrote:Discones like the one above are excellent on VHF/UHF because they are wideband antennas.

The ones I linked to earlier. The one ChrisP made is a variation on the discone & equally good on broadband & having more metal in the air, probably better for weaker signals too.


Thanks

Is that the bi-cone or the bow-tie?

When you say 'wideband' how wide is that? If i want to cover 50Mhz to 500Mhz well, do i need 1, 2, 3 etc to get the best reception?
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Re: Getting into scanning/Listening - antenna help.

Postby m0lsx » Thu Sep 05, 2019 4:33 pm

The Bicone & the Bowtie both come from the same roots. They are both wideband antennas, with the bowtie basically just being a simplified version of the bicone.

Both should give several octaves of coverage.. So keeping things simple, an octave is a ratio of 2:1.. So at 100MHz, 1 octave up is 200Mhz & one octave down is 50MHz. So double & half & thus a very broadband antenna.

Personally I would start with a bowtie as it is such a simple antenna & then try a Bicone, as it should be slightly better as it puts more metal in the air & thus should offer slightly better reception.
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Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Thu Sep 05, 2019 5:06 pm

I'll have to dig out some lumps of wood or something - a bowtie for 100Mhz is pretty unwieldy - 2.5m wide x 0.75m high, calc shows a bandwidth of only 33Mhz though - presuming thats for transmitting?
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Re: Getting into scanning/Listening - antenna help.

Postby Chris P » Fri Sep 06, 2019 9:41 am

viewtopic.php?f=4&t=5662&p=30306#p30292 shows details of my bowtie for airband use, it is much smaller than the dimensions you get from the calculator and covers both civil and mil airbands and the marine band



Note that for most of the users in the 50 -500Mhz range vertical polarisation is preferred so the bowtie should be mounted with its elements vertical so that it looks like an X rather than ><
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Re: Getting into scanning/Listening - antenna help.

Postby Dave In Herts » Fri Sep 06, 2019 4:49 pm

I have three now - a slim-jim, a coaxial dipole and the bowtie - all have reasonably similar results but the slim-jim wins a little provided its kept away from the house.

If your calculations a shorter, how were they arrived at?



Thanks

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

Postby Chris P » Fri Sep 06, 2019 10:42 pm

The dimensions I used were provided by other experimenters on Fighter Control forum and further research by myself . I found a technical paper ( which I no longer have )explaining the theory around the bow tie 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 . This length also kept the impedance of the antenna close to a match to a 50 ohm feed again over the same range .


I have found that the slim jim does have a small amount of gain over a narrow bandwidth around its design frequency compared to a dipole ,bicone and discone but this is lost as you get further away from the centre frequency . I have recently tried a bazooka antenna on VHF Airband which is also similar in performance the dimensions and constructional details can be found on merseyradio.co website . The website gives constructional details for both a VHF and UHF airband antenna so may be of interest to you .
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