Discone recommendations?

Useful information regarding antennas for SDR products.
franko
Posts: 15
Joined: Mon Aug 28, 2017 9:33 pm

Discone recommendations?

Postby franko » Wed Sep 06, 2017 12:07 am

After a week or so with my new SDRplay RSP2, I've been getting decent results in HF with an end-fed long-wire antenna. I've also ordered an active mini-whip to see how that compares.

I'm now thinking about a general-purpose antenna for VHF and above, to be used at my home location on one of the SDRplay's ports. There is no one particular band that I'm focused on -- I'd like to listen to amateur traffic on 2 meters, but also marine radio traffic (if I can get it as far inland as I am), perhaps check out various public agency scanner frequencies, etc. I'm open to anything out there that would be interesting to receive.

I've seem discone antennas mentioned favorably for these frequency ranges. Are there any particular ones made commercially you would recommend, hopefully ones that would be fairly easy for me to obtain in the U.S.? Or if this is a not-too-demanding design to do as a homebrew, are there any good instructions out there?

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vk7jj
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Joined: Thu Mar 02, 2017 9:56 am

Re: Discone recommendations?

Postby vk7jj » Wed Sep 06, 2017 4:28 am

I have two, both different brands and sourced two years apart, yet are both identical.

They look exactly the same as 90% of those I've seen at fellow ham shacks, they all look like this:

http://www.ebay.com/itm/SIRIO-SD-1300-D ... 2424956113

As per the advert the top section is tuned to the 50MHz ham band, without that they cut off well above 50MHz though I can't remember the roll off frequency.

You'll need coax with an N connector to fit that particular one, as per it's specs.

They perform very handily on receive, I don't have professional level tools but I've swept mine using a RigExpert AA-1400 (0.1 to 1400MHz) and it shows the SWR is not quite respectable enough to use for a full time transmit antenna but that's irrelevant for an RSP.

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franko
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Joined: Mon Aug 28, 2017 9:33 pm

Re: Discone recommendations?

Postby franko » Wed Sep 06, 2017 5:00 am

Thanks for the information and the link, that looks pretty much like the examples I've seen online.

Do you have any impressions on how the antenna's performance (for receiving only) is affected by the length of the lead-in coax? While I can probably work out mounting it fairly close to my receiver, I think I'd get a better location if I place it somewhat further away. Do you have any thoughts on what kind of maximum lead-in length would be appropriate for a given type of coax?

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vk7jj
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Joined: Thu Mar 02, 2017 9:56 am

Re: Discone recommendations?

Postby vk7jj » Wed Sep 06, 2017 6:27 am

The easiest thing is to work out in your own mind the best compromise between the cost of various grades of coax and connectors V's the signal loss at the frequencies you are interested in over the length of the coax run.

There are lots of online coax loss calculators that work the loss out for you, eg.

http://www.timesmicrowave.com/calculato ... =calculate

Depending on your ability with a soldering iron or crimpers, if you were faced with a long run then a relatively widely available coax like RG-6 would be tempting, it's flexible and cheap and even though it's 75 ohms rather than 50 that would make no discernible difference to your situation imho. The main downside is that it is hard to connect to as it has a thin metal shield under the braid.

Often people give up and use low loss RG58 because it's simpler and RG213 is a bit solid and stiff and expensive to buy and install just for casual listening. How about having a chat with your local hobby shop, lots of people have the same problem with scanners and techno/ham hobby shops will often suggest an option or two for you.

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franko
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Joined: Mon Aug 28, 2017 9:33 pm

Re: Discone recommendations?

Postby franko » Wed Sep 06, 2017 6:55 am

Thanks, I'll check out the coax loss calculator.

I did actually bring this topic up with the local shop. Their advice was (a) I'm probably best off sticking with my end-fed long-wire antenna with a plain-wire lead-in across all bands up into VHF and beyond, and (b) coax is really only necessary for transmitters, not receivers.

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vk7jj
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Joined: Thu Mar 02, 2017 9:56 am

Re: Discone recommendations?

Postby vk7jj » Wed Sep 06, 2017 7:18 am

Good on you; sometimes asking creates a problem, quot capita tot sensus and I shudder to think of all the opinions being roused right now amongst this readership, the antenna topic is endless.

a) any appreciable length of end fed wire is going to be a very noisy antenna. I've never ever tamed one and I'm pretty good ;-) I currently have two seperate 160m long tuned loops and my joy is antenna modelling.

To use an end fed wire on VHF is to invite very strong AM broadcast band signals being present at your input all the time. Go for the discone. Save you connecting and disconnecting the antenna or switching it between the Hi-Z input and A or B just so you can use the broadcast band notch filter.

b) absolutely true, cheap TV ribbon has marvellously low loss compared to coax as the frequency rises and is much more forgiving even when run along the floor than the purists who insist on complex insulating systems will admit. We've been coerced into using coax by the adoption of 50ohm coaxial connectors by the manufacturers of the first solid state rigs.

The main problem with ribbon is connecting it to your discone as that has a coax connector inside a metal sleeve.

I personally use an inexpensive twisted teflon pair for several antennas including one of those loops, teflon because I transmit on it; I bought cheap teflon out of the states on ebay and have spent quite some time measuring the performance and impedance. I just mention that in case you have something lying around.

PS. I made the 160m loops out of aluminium welding wire, cheap and plentiful and low resistance, birds fall off it, easy to keep in the air; you could consider a modest loop?

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g1hbe
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Location: Cheshire

Re: Discone recommendations?

Postby g1hbe » Wed Sep 06, 2017 8:04 am

I use CT100-type satellite cable for all my lead-ins as it is cheap, low-loss up to GHz and flexible. It's a 7mm diameter cable, but with a bit of ingenuity you can fit normal connectors. I tend to terminate in the shack with satellite F-connectors and then adapt from there.

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Andy

franko
Posts: 15
Joined: Mon Aug 28, 2017 9:33 pm

Re: Discone recommendations?

Postby franko » Wed Sep 06, 2017 4:53 pm

Thanks for the thoughts. I suspect the shop staff was just trying to save me some money (lash something together, hear something) while I get my feet wet and see where my interests lie.

Vk7jj, I've in fact found my end-fed wire to be pretty noisy in VHF, so I do see a discone in my near future. G1hbe, thanks for the suggestion of CT100. I'll look around and see what cables are reasonably priced here and see if I can find something to work with as a starting point. I can easily see antennas being an area in which I'd enjoy doing a lot of experimentation.

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franko
Posts: 15
Joined: Mon Aug 28, 2017 9:33 pm

Re: Discone recommendations?

Postby franko » Sun Sep 10, 2017 12:45 am

Update: Thinking I would try out a discone antenna while keeping the cost minimal, I ordered a Workman T-601 (~US$33) which arrived today. This is a relatively compact Taiwanese-made model that is marketed by at least a couple of companies (besides Workman, the identical antenna is also sold as the Jetstream JTD1). It also seems to have decent reviews. The stated reception coverage is 25-1300 MHz.

After quick assembly, I mounted it on the end of a PVC pipe about 4 meters in the air outside my window, and connected it to the SDRplay. All I can say is wow -- signal strength and definition are light-years beyond that achieved by the random-length end-fed long-wire I'd been playing with initially. So far I've tested the discone on the commercial FM band, our local NOAA weather stations, and some of the area scanner frequencies for public agencies (many of which, however, appear now to be trunked and/or encrypted). I'll have to do some more digging around to look for frequencies to check out. It would be neat if the SDRplay itself could be placed in a scanning mode to range across VHF and UHF to see what pokes up out there.

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