Discone questions

[paulmohr]

Seriously considering getting a discone for my scanner. I did some research and searched the threads. TONS of info on them. So much so that it was confusing to digest and really didn't get me anywhere lol. So have a few questions of course.

1) is there any particular style, design or brand I should stay away from. I see they come in various designs and prices that range from cheap to stupid expensive. Lowest frequencies I really need to cover are around 100mhz. So don't need one that goes down to 20mhz or something. And I obviously don't want to pay 200 dollars for something that doesn't work any better than one I could get for 50 bucks, just because of the brand name. And transmitting will never be an issue, this is receive only for my 325p2 scanner.

2) Would it hurt anything if I ran 50 ohm coax with the connector designed for the antenna down to my grounded 75 ohm connector block I already have? I am sure they make an adapter, or I can just cut it to the right length and put my own connector on it.

3) Would building my own be any better or worse than just ordering one? I downloaded a program that shows you to build one and calculates all the lengths and stuff. Looks like it would be fairly easy to build out of simple fencing material. Or if I wanted to get really fancy I could use copper or something. Parts would probably cost me about as much as ordering a cheap one though (50 dollars or so).

 

[majoco]

The difficult part in making your own, unless you have a nice lathe and a big block of aluminium, is making the centre piece and getting the angles right. The distance between the "top hat" and the radials is critical and the angle of the radials determines the impedance. The cheap ones are relatively fragile but probably perform OK in the short term but will they last a couple of winters???

Mine was rescued from the scrap heap when one of our remote offices closed down and the building (shed?) was demolished for another operator - I got there just in time and that was 30years ago! Solid cast ali centre, thick perspex insulator and half-inch tubing bolted on - the PL259 now has a plastic sleeve over it to keep it dry. I had to fill the tubing up with builders foam to stop it organ-piping - kept the XYL awake!

 

[mmckenna]

Seriously considering getting a discone for my scanner. I did some research and searched the threads. TONS of info on them. So much so that it was confusing to digest and really didn't get me anywhere lol. So have a few questions of course.

1) is there any particular style, design or brand I should stay away from. I see they come in various designs and prices that range from cheap to stupid expensive. Lowest frequencies I really need to cover are around 100mhz. So don't need one that goes down to 20mhz or something. And I obviously don't want to pay 200 dollars for something that doesn't work any better than one I could get for 50 bucks, just because of the brand name. And transmitting will never be an issue, this is receive only for my 325p2 scanner.

I don't know which brands you should stay away from, but I think sticking with a known name brand is pretty safe. I've got a Diamond I rescued a few years ago that's working well, no issues.
I've also got a $1500.00 one at work, and while it functions well and will outlast world war 3, there's no need for a hobbyist to spend that much on an antenna.

Do stick with a name brand. I'm personally wary of Browning/Tram after looking at their products at a trade show and talking to the rep.

Go with the N connector models if you can.

Make sure you install it properly.

2) Would it hurt anything if I ran 50 ohm coax with the connector designed for the antenna down to my grounded 75 ohm connector block I already have? I am sure they make an adapter, or I can just cut it to the right length and put my own connector on it.

Won't hurt anything. Keep in mind the issues with long cable runs, etc.
Ideally you do want a continuous run of high grade coax, but that can come later on.

3) Would building my own be any better or worse than just ordering one? I downloaded a program that shows you to build one and calculates all the lengths and stuff. Looks like it would be fairly easy to build out of simple fencing material. Or if I wanted to get really fancy I could use copper or something. Parts would probably cost me about as much as ordering a cheap one though (50 dollars or so).

Not impossible to build you own, but unless you have a small machine shop, it's going to be difficult. The key is getting insulating material that will stand up to years/decades of UV exposure.
The manufacturers have already figured this all out, and can probably do it cheaper than you can.
Building your own antennas can be rewarding, but there are easier designs to start with.
If you need a discone now, buy a reasonably priced known name brand and enjoy it.

 

[spongella]

Discones usually cover a very wide frequency range, e.g. 25 MHz - 3 GHz such as the one I am using. If you only need 100 MHz coverage you may want another antenna type such as a simple scanner antenna; what frequencies are you looking to receive? Do you need 100 MHz range, or are you interested in receiving 100 MHz (FM BCB)?

 

[paulmohr]

The one I was thinking of building doesn't really look like the commercial disones. The plans I have the antenna looks more like a lamp shade than a big metal spider. There is a top hoop, and a bottom hoop. If you know length of the side elements and distance between the top and bottom hoop the angle takes care of itself. No need for the large aluminum piece with angle cut into it. Well you also have to have the diameter of the hoops correct as well lol.

I sort of priced some of the materials and tools to make one and it won't be as cheap as I thought after all.

If I were to build one it would look more like this:
http://cdnimages.opentip.com/full/TES/TES-396375.jpg

That is a 6,000 dollar antenna though LOL. It would not be exactly like that, but a similar design.

Specific frequencies I need are 155, 450 and 800. The 800mhz tower is pretty close so that is not a concern at all for me. If I were to build one I would probably make the cut off 100mhz just to keep the size from getting crazy large.

 

[paulmohr]

Won't hurt anything. Keep in mind the issues with long cable runs, etc.
Ideally you do want a continuous run of high grade coax, but that can come later on.

Don't I need to ground the cable before it comes into the house? That was point for connecting to the grounding block connector. I already have one installed with the cable running in the house, but its an F connector with RG6 cable running into the house.

If it doesn't need to be grounded I could just as easily order a cable with a bnc connector on one end and hook it up directly from the antenna to the scanner. Down side is I have to drill another hole in the house to feed the line through. And it isn't my house until she dies lol.

 

[mmckenna]

If I were to build one it would look more like this:
http://cdnimages.opentip.com/full/TES/TES-396375.jpg

That is a 6,000 dollar antenna though LOL. It would not be exactly like that, but a similar design.

That is similar to the one I have at work. Nice antenna, but not $6000.00 nice. The price comes from the durability. When you stick on of those on top of a tower and expect it to last for 10-20 years without any issues, it costs.

Specific frequencies I need are 155, 450 and 800. The 800mhz tower is pretty close so that is not a concern at all for me. If I were to build one I would probably make the cut off 100mhz just to keep the size from getting crazy large.

A discone would work for that, but so would a simple multi-band vertical.

 

[mmckenna]

Don't I need to ground the cable before it comes into the house? That was point for connecting to the grounding block connector. I already have one installed with the cable running in the house, but its an F connector with RG6 cable running into the house.

If it doesn't need to be grounded I could just as easily order a cable with a bnc connector on one end and hook it up directly from the antenna to the scanner. Down side is I have to drill another hole in the house to feed the line through. And it isn't my house until she dies lol.

It does need to be grounded, but there are several ways to do that.
The grounding blocks that are often used for TV antennas are not necessarily suitable for these sorts of antennas since they ground the outer shield only.
You need to protect the outer shield and the inner conductor. Using a product like a PolyPhaser would be the recommended industry standard type protector.

Still, a grounding block is better than what most people use.

 

[allend]

Go with a Diamond D130J with an N connection to F female connector and run RG-6 to your scanner.

You won't regret it. You can do this for under 200 bucks and be done with it and be extremely happy.

You don't need the LMR400 feed line. Just buy a box of RG-6 and some compression fittings and its a solid as can be.

 

[paulmohr]

Ordered, one D130J and a UHF to F coax adapter. Hopefully they will be here by the weekend. Coming from two different places through Amazon though, so who knows lol. Now I just need to figure out where to mount it. If I mount it to the tower on the other side of the house I can get a bit higher, but the cable run would be about twice the length, maybe 50 feet or so. If I mount it to the unused chimney on the east side of the house it is pretty much a straight shot down to my room in the basement. Probably only a 10 foot height difference between either one.

If I were to put it on the tv tower, would the large TV antenna already up there interfere with the discone reception?

Thanks for the help everyone, I was considering ordering one of those generic 40 dollar ones, you guys talked me out of that idea. I think I could probably build a pretty nice one if I wanted to. It wouldn't have the lower frequency coverage though, would have probably cost me just as much in parts and tools and would probably be more heavy ( the way I was going to build it anyway). Not to mention it would like I mounted a large bird cage to the top of my house lol.

 

[mmckenna]

Ordered, one D130J and a UHF to F coax adapter. Hopefully they will be here by the weekend. Coming from two different places through Amazon though, so who knows lol. Now I just need to figure out where to mount it. If I mount it to the tower on the other side of the house I can get a bit higher, but the cable run would be about twice the length, maybe 50 feet or so. If I mount it to the unused chimney on the east side of the house it is pretty much a straight shot down to my room in the basement. Probably only a 10 foot height difference between either one.

Depends on what you are trying to listen to.
Since these frequencies are generally line of sight, the height of the antenna will dictate how far off the radio "horizon" is.
If what you want to listen to is between your antenna and the horizon, then you should be able to hear it.
There are online "distance to horizon" calculators that will help you figure this out.

Where the tower location will impact things is that extra length of cable. If the signal you are trying to pick up is really weak, then it might get lost in the inherent feed line losses.

If I were to put it on the tv tower, would the large TV antenna already up there interfere with the discone reception?

No, the TV antenna won't interfere with the scanner antenna. Ideally you do want the separated.

Thanks for the help everyone, I was considering ordering one of those generic 40 dollar ones, you guys talked me out of that idea. I think I could probably build a pretty nice one if I wanted to. It wouldn't have the lower frequency coverage though, would have probably cost me just as much in parts and tools and would probably be more heavy ( the way I was going to build it anyway). Not to mention it would like I mounted a large bird cage to the top of my house lol.

I think you made a good choice.

 

[paulmohr]

Well we can scratch all that, for some reason amazon emailed me and cancelled both orders because of an "issue" with my debit card. So I get to go through this crap again. But it shows they charged me a dollar just for the heck of it lol. I am looking at my bank account right now and there is more than enough in there to cover what I ordered so I don't know what the issue is.

DX is in ohio at the summit racing complex, about 3 hours away from me. Maybe I should just drive there and pay freakin cash for it.

 

[mmckenna]

If you do go to pick one up, and have the choice….
Preference would be for the model with the N connector.

N connectors have slightly better performance than the UHF type connectors. Unlikely you'd ever notice it in the quality of the signal.
The other benefit of an N connector is that they are designed to be waterproof whereas the UHF are not.
While you shouldn't rely on that instead of proper waterproofing, it's a good thing to have.

Either way, I think you'll be happy with that antenna. Worth a trip to DX just to do the window shopping. Who knows what else you'll see while there. Something to be said for brick and mortar stores where you can actually touch the stuff before buying it.

 

[mmckenna]

Diamond D130NJ is the model number.

 

[paulmohr]

I ordered one from a company Universal Radio in columbus ohio. I got the N model with a n to f adapter. We will see what happens, it looks like kind of a small company so it might take a few days for them process the order.

 

[mmckenna]

Universal Radio is has been around for a long time. I suspect you'll get good service from them. Maybe not as fast as Amazon, though.

 

[Ubbe]

Discones usually cover a very wide frequency range, e.g. 25 MHz - 3 GHz such as the one I am using.

The lower radials dictates the lowest 1/4 wavelenght frequency of the discone. Some scanner discones have a vertical element added to the top and is a 1/4 wavelengt for a single frequency and always have a shortening coil that make it impossible to judge the frequency by eye.

Most discones are 100MHz-1000MHz (30 inch lower element) and that vertical top element are often tuned to 35MHz.

/Ubbe

 

[spongella]

Universal Radio is a great company, to deal with, good choice. Wrapping your connection at the antenna with a product like Coax-Seal will also help keep moisture out. Enjoy your new antenna.

 

[rfradioconsult]

Biggest problem is the screw in elements vibrating loose and falling out, a good idea is some sort of thread lock applied when assembling the antenna.

 

[ridgescan]

Biggest problem is the screw in elements vibrating loose and falling out, a good idea is some sort of thread lock applied when assembling the antenna.

My D130J has been up there since 2004 and never had this problem you speak of.