VHF / UHF antenna choice for RTL SDR

[ScottBouch]

Hi all,

Following on from my previous question about VHF/UHF scanners/receivers, i have just ordered an RTL SDR USB dongle.

I bought with it a cheap telscopic SMA antenna to just get me started and hopefully be fine for local use.

I am mainly interested in hearing aviation VHF and UHF on 117.50MHz to 135.95MHz and 225.00 to 399.95Mhz. I have a pair of aircraft skin mounted blade antennas, inteneded for these ranges respectively, so could start by trying these, given a ground plane to mimic the aircraft fuselage.

But what other options are there for a single broad band antenna for both ranges? Happy to make something up like a whip with groundplane prongs etc.. As you will be able to tell, i'm trying to learn about antennas!

Also of interest is trying to listen to amateir radio bands. I've seen people with massive loops round their gardens, not sure wifey would approve of this, so what other options are there for SDR listening only?

Cheers, Scott

[Radio Wrangler]

A discone is the usual solution to this problem. Very broadband, good impedance from a min freq set by the length of the rods up to a max freq set by how finely the junction of the disc with the cone is made. Good impedance match as well.

Commercially made ones available from amateur radio distributors. DIY drawings in various amateur radio handbooks.

Looks like a chimney sweep's brush (Made of rods) on its pole with an added skirt of rods as a cone descending and outwards below the 'brush' centre.

David

[G6Tanuki]

For VHF listening [50MHz and up] I would suggest a Discone; they cover a wide frequency-range [mine here is 40 to 500MHz so it covers the 6 and 4M amateur bands, Band II broadcast, civil airband, 2M amateurs, DAB and the 225-400MHz military airband].

There are numerous designs for Discones available out there; you can easily make one using a fistful of brazing-rods and a ;hub' insulator cast from bits of plastic waste-pipe filled with Araldite, Plastic Padding or similar.

[ex seismic]

For the mil band you might look for either Larkspur or Clansman era discones. The Larkspur one looks like a normal discone, the Clansman one was known as the "Bob Marley" you will see why if you find one.

Gordon

[ScottBouch]

Thank you guys, much appreciated!

When I wrote whip antenna with ground plane spikes, I had the mental image of a discone, bit no idea what it was called!

Lots to think about there, thank you for the pointers!

[ScottBouch]

Haha, I like the Bob Marley antenna!

[Cruisin Marine]

If you choose a Discone, try to keep the coax length to a minimum, using the best cable you can. Losses can be terrible at UHF with those antenna.

[Radio Wrangler]

The cable loss isn't affected by the antenna being a discone, it's just that the discone being omnidirectional has no directivity and hence no effective gain to help make up for cable loss. I put a discone on top of the HP microwave building with 100m of Westflex H100 feeder down to R&D, it proved useful. When R&D moved floors, it was re-done with Heliax which improved things several dB.

Amusingly, years later when there were changes afoot to the disadvantage of people working there, the managers held a private meeting in the canteen building and used radio microphones (174MHz) In a factory producing spectrum management receivers and spectrum analysers! Oh, dear.

David

[Cruisin Marine]

Originally Posted by Radio Wrangler

The cable loss isn't affected by the antenna being a discone, it's just that the discone being omnidirectional has no directivity and hence no effective gain to help make up for cable loss. I put a discone on top of the HP microwave building with 100m of Westflex H100 feeder down to R&D, it proved useful. When R&D moved floors, it was re-done with Heliax which improved things several dB.

Amusingly, years later when there were changes afoot to the disadvantage of people working there, the managers held a private meeting in the canteen building and used radio microphones (174MHz) In a factory producing spectrum management receivers and spectrum analysers! Oh, dear.

David

lol

[ScottBouch]

One potential solution to the cable length issue - as this is a USB device, could use a USB extender placing the SDR at the antenna and keeping the RF side very short...

But on this topic of RF cables, does 50 Ohm / 75 Ohm impedance cable make much differenece to a receive only RTL SDR setup?

Also regarding discone antennas, there has been mention of 'good impedance', but what does this mean in terms of ball park numbers?

[ScottBouch]

PS, the RTL SDR datasheet says: Typical input impedance = 50 Ohms.

But again this is a little vague as it includes the word typical, so is not a specification..

[Jon_G4MDC]

My SDR is in the shed at the end of the garden and I use it from the house over wifi.

[ScottBouch]

Originally Posted by Jon_G4MDC

My SDR is in the shed at the end of the garden and I use it from the house over wifi.

Do you use a Raspberry Pi or similar to provide the network stream?

I have Ethernet all over my house, this would be another good option.

[Jon_G4MDC]

That would be one way. In my case it's an old desktop PC running Windows 7.
It has one dongle for HF and another for VHF/UHF.

[ScottBouch]

REF the Clansman PRC 344 Bob Marley Discone antennas - are these designed for UHF only? ie; do they cover a broad spectrum like other Discone antennas?

I ask as online any pages on them only mention UHF.

[Radio Wrangler]

All discone antennae are fairly wide band. How low they go is a function of overall diameter. The last one I bought had an added whip on top extending it down to the 10 metre band. It was a Weltz-Diamond make. Have a look at Martin Lynch & son website, or that amateur radio shop in Barnsley.

How high they go depends on the accuracy at their throat, where the disc and the cone come together. The angle of the cone sets the imedance.

The other broadband antenna is the log-periodic. Usually a bigger beastie and directional, so one of these would need to go on a rotator.

Both approaches lead to a quite satisfactory approach to 50 Ohms, good enough for transmitting. Most receivers have input impedances rather far from what gets specified so matching is a bit of a joke. RF transistors give their best noise performance and their best gain performance with quite different effective source impedance and you can't have both at once. This has been driving designers up the wall for decades. Just don't worry about it.

David

[G6Tanuki]

The Bob Marley is kinda optimised for the military airband, 220-400MHz and will be rather less efficient at lower frequencies.

One thing to consider with Discones is that their broadband abilities can be a nuisance if you live close to a powerful transmitter. If you are trying to listen to a weak civil airband station your antenna's good response at the lowest end of Band-II broadcast can mean your receiver gets swamped by the multiple transmitters each emitting 100Kilowatts of the BBC national stations.

Thankfully, if you are troubled by such intrusive stuff they can be taken out using coaxial stub filters.

[ScottBouch]

Really sound advice chaps, thank you.

This is a big learning curve for me, I really appreciate the inputs here, helping me to learn.

Thanks, Scott

[Radio Wrangler]

As computers tend to be broadband noise emitters, having computation (including the SDR) away from the antenna is attractive. If necessary add a nice little broadband amp at the antenna end. It can be phantom powered up the coax.

That was my plan 'B' if the 100m length of heliax hadn't worked.

David

[ScottBouch]

Ah, thats another good point, thanks. So mounting the antenna directly to the SDR, plugged directly into a laptop USB socket would be far from ideal regarding unwanted noise!

At home we aso have a PV solar array with optomisers (which communicate over the DC power wires) battery and inverter, I'm thinking the remote connection to the shed is possibly a better idea than using the house roof!