Radio Shack Discone question

[crippledchicken]

I just purchased this antenna and at this time, most everything in my area is between 154 and 156mhz range. the main vertical element is 28" long would I be better off to cut this length closer to around 19"? I know the antenna book they sell, suggests around this length for best performance on their ground plane antenna project. Thanks!

 

[JASII]

Discone

The discone itself is wide band and has unity gain from say 100 mHz to 1 gHz. I think they added the long low band element for say 25 to 54 mHz. I honestly doubt that it would make much difference on VHF high band. If I were you I would leave well enough alone.

 

[Highlander_821]

You can actually trim that whip to do 6 Meter Ham, but I don't remember the length we ended up with. My friend was on the roof making the adjustments while I was in the shack with the transmitter and SWR meter.

If all you are doing is monitoring, I would just leave it at the factory length.

 

[kb2vxa]

DON'T cut that element if you want to receive VHF Lo Band! Hams who wish to use the antenna to transmit on the 6M band may do so because they have the knowledge and equipment to make proper adjustments, you don't.

Some "discones" such as this one are hybrids, a cross between a discone and a ground plane. The vertical element works against the longest sloping elements, there's your ground plane for Lo Band, the rest is a discone from about 100MHz upwards. To clarify, the name implies disc and cone, the disc sitting atop the cone. They need not be solids, it's the geometry of the antenna that gives it it's name.

 

[45SigSauer]

Just a little word of advice, once you have it all put together, put a little dab of silicone around where the elements screw in, it will help to weather proof it, and more importantly it will help to keep the elements from coming unscrewed and falling off.

 

[hoser147]

As the rest have said leave it alone it will work fine. I have the same discone works great for me........

 

[prcguy]

Warren is correct but if you will never use it below 100MHz just leave the top whip off as it has a small effect on the radiation pattern at other frequencies.
The whip has a loading coil at the base to make it resonate around 50MHz and the coil also acts as a choke to minimize the whip interaction at higher frequencies where the Discone takes over. If you were to attach a whip with no loading coil in the same spot, the top whip would radiate undesirable patterns as it becomes longer than about 3/4 wavelengths at higher freqs and it can also cancel out the desirable pattern from the Discone side.
prcguy

DON'T cut that element if you want to receive VHF Lo Band! Hams who wish to use the antenna to transmit on the 6M band may do so because they have the knowledge and equipment to make proper adjustments, you don't.

Some "discones" such as this one are hybrids, a cross between a discone and a ground plane. The vertical element works against the longest sloping elements, there's your ground plane for Lo Band, the rest is a discone from about 100MHz upwards. To clarify, the name implies disc and cone, the disc sitting atop the cone. They need not be solids, it's the geometry of the antenna that gives it it's name.

 

[crippledchicken]

Well, too late! by the time I read the posts I had already cut it to 19" and mounted it. I replaced one just like it that I had for 12 years so, if I need to I can use the element from the old antenna to get full length again but I can't tell any difference also, I can use any of the other elements,parts, etc. if needed later on. Thanks for all the wisdom, and advice!

 

[ka5lqj]

TRUE "disk cone"......

According to my 1953 "Editors and Engineers Handbook", the "disk" of the discone is 70% of a half-wave length of the lowest frequency you wish to monitor. It is made up of a sheet metal disk or wires horizontal to the terrain. There is NO "vertical" element. The ground radials stick out from the grounded insulator at a 45º angle. This "insulator" can be made from plexaglass or wood in a "doughnut" shape to mount in a piece of 1-3/8" fence top rail, with the top being solid and a small hole drilled to mount the horizontal disk. The center conductor of the coax connects here with a soldering lug to the underside of the disk and the bolt or screw on top with a generous supply of silicone.

The underside of this is drilled and as many as possible ground radials are added at a 45º angle. These are all bonded together with solder wick and soldered and "glued in" with silicone. The "shield" of the coax attaches here, soldered and insulated with silicone to prevent water from getting into the coax. ;-)

Even though the true discone, LQ^QKS like a horizontally-polarized antenna, it's a vertical...confusing, I know

Radio $hack's version of the "Discone", is more along the lines of a "Ground Plane" with the lower radials bent to a 45º to lower the antenna's impedance to 50 ohms. If the lower elements were left straight out, parallel to the terrain, the antenna impedance would be closer to 75 ohms.

A Discone and Ground Plane have "0" gain or less. If you live close by or don't want to pick up the pager systems, these are a wise choice. However, there ARE antennas you can build with wire and PVC that will do a better job of longer distance reception (and transmission, if you're a HAM).

http://www.hamuniverse.com/g2bcxslimjimantenna.html Be SURE to silicone the hot and ground-side leads on the coax for wx protection. ;-)

 

[prcguy]

The disk can be between 63% and 70% of a quarter wavelength (not a half wavelength) at the lowest operating frequency. Bending the radials of a ground plane down at a 45deg angle RAISES the impedance, not lowering it as mentioned below.
A Discone is an "aperture" antenna and launches a vertically polarized wave from the apex of the disc and cone.
prcguy

According to my 1953 "Editors and Engineers Handbook", the "disk" of the discone is 70% of a half-wave length of the lowest frequency you wish to monitor. It is made up of a sheet metal disk or wires horizontal to the terrain. There is NO "vertical" element. The ground radials stick out from the grounded insulator at a 45º angle. This "insulator" can be made from plexaglass or wood in a "doughnut" shape to mount in a piece of 1-3/8" fence top rail, with the top being solid and a small hole drilled to mount the horizontal disk. The center conductor of the coax connects here with a soldering lug to the underside of the disk and the bolt or screw on top with a generous supply of silicone.

The underside of this is drilled and as many as possible ground radials are added at a 45º angle. These are all bonded together with solder wick and soldered and "glued in" with silicone. The "shield" of the coax attaches here, soldered and insulated with silicone to prevent water from getting into the coax. ;-)

Even though the true discone, LQ^QKS like a horizontally-polarized antenna, it's a vertical...confusing, I know

Radio $hack's version of the "Discone", is more along the lines of a "Ground Plane" with the lower radials bent to a 45º to lower the antenna's impedance to 50 ohms. If the lower elements were left straight out, parallel to the terrain, the antenna impedance would be closer to 75 ohms.

A Discone and Ground Plane have "0" gain or less. If you live close by or don't want to pick up the pager systems, these are a wise choice. However, there ARE antennas you can build with wire and PVC that will do a better job of longer distance reception (and transmission, if you're a HAM).

http://www.hamuniverse.com/g2bcxslimjimantenna.html Be SURE to silicone the hot and ground-side leads on the coax for wx protection. ;-)

 

[crippledchicken]

Warren is correct but if you will never use it below 100MHz just leave the top whip off as it has a small effect on the radiation pattern at other frequencies.
The whip has a loading coil at the base to make it resonate around 50MHz and the coil also acts as a choke to minimize the whip interaction at higher frequencies where the Discone takes over. If you were to attach a whip with no loading coil in the same spot, the top whip would radiate undesirable patterns as it becomes longer than about 3/4 wavelengths at higher freqs and it can also cancel out the desirable pattern from the Discone side.
prcguy

Since I will never, be receiving anything below the 150mhz range would it actually be better performance wise, to just remove the whip? Thanks!

 

[Mike_G_D]

Since I will never, be receiving anything below the 150mhz range would it actually be better performance wise, to just remove the whip? Thanks!

The short answer is yes. As others have tried to explain, a true discone does not have a vertical whip. Unfortunately, to put it simply, a true discone covering 25MHz and up would be quite large and mechanically impractical for most installations. So the hobbiest manufacturers "cheat" by sticking a "low band whip" on the top of the discone which, ideally, makes it perform, more or less, like a basic ground plane antenna at the lower frequencies (though the ground plane radials are somewhat short at those frequencies it is better than nothing, at least for receive only usage). The RS discone and others like it are generally designed to perform as a true discone starting at about 100MHz or so. So, in your case, yes, just remove the low band whip and maybe put some silicone sealer on the mounting screw for weather protection and to preserve the mounting screw for future use in case you ever need it or you intend to sell the antenna.

-Mike

 

[crippledchicken]

Thanks for the info. I'm not much of an antenna whiz as you can tell but, have quite a bit of experience with CB radio repair, tweaking and peaking, adding channels and also audio component repair and upgrades etc. I've been into electronics for over 30 years but antennas, the most experience I had was setting SWR's and such for CB's and and some of the 10 meter ham. Thanks again I'm always looking to learn all I can being retired I have too much time on my hands to be waisting.

 

[kb2vxa]

Cripple, never reveal your illegal activities especially when they encourage others to break the law. I don't give a rat's rectum what you idiots do on CB but stay off our 10M band unless you're looking for trouble; ham bands re NOT free bands, they come with a high price for interlopers. Now you know that by posting you have crossed into enemy territory so behave yourself and use that time on your hands for something constructive rather than polluting the radio spectrum.

 

[prcguy]

Wouldn't it be embarrassing (for you) if the guy has a ham license.....
prcguy

Cripple, never reveal your illegal activities especially when they encourage others to break the law. I don't give a rat's rectum what you idiots do on CB but stay off our 10M band unless you're looking for trouble; ham bands re NOT free bands, they come with a high price for interlopers. Now you know that by posting you have crossed into enemy territory so behave yourself and use that time on your hands for something constructive rather than polluting the radio spectrum.

 

[crippledchicken]

Cripple, never reveal your illegal activities especially when they encourage others to break the law. I don't give a rat's rectum what you idiots do on CB but stay off our 10M band unless you're looking for trouble; ham bands re NOT free bands, they come with a high price for interlopers. Now you know that by posting you have crossed into enemy territory so behave yourself and use that time on your hands for something constructive rather than polluting the radio spectrum.

Sir, let me tell you it has been many years ago and, I don't give a rats ass what you think. put that in your pipe and smoke it!

 

[crippledchicken]

Wouldn't it be embarrassing (for you) if the guy has a ham license.....
prcguy

I don't but I mean my gosh! I don't think I said anything to deserve a personal attack like that. I have been on this forum for several years and have always been polite to everyone maybe it's time to move on I guess.

 

[prcguy]

Since 10m is the only amateur band the attacker has HF phone privileges on, its no wonder he is fussy about CBrs encroaching on it. You can't take anyone too seriously here, don't be afraid to share your thoughts or experiences.
prcguy

 

[hoser147]

I don't but I mean my gosh! I don't think I said anything to deserve a personal attack like that. I have been on this forum for several years and have always been polite to everyone maybe it's time to move on I guess.

Just blow it off, no reason to leave. Its a forum, ya got have a tough skin. Been a good thread.

 

[kb2vxa]

"Wouldn't it be embarrassing (for you) if the guy has a ham license?"

Fixed it for ya. No it wouldn't, back in my CB daze we had a ham and holder of a P2 license doing illegal mods; I didn't approve of it then and I don't approve of it now.

"Sir, let me tell you it has been many years ago and, I don't give a rats ass what you think. put that in your pipe and smoke it!"

Typical attitude of an alpha hotel and I have better things to smoke too.

"I don't think I said anything to deserve a personal attack like that."

That's exactly why you got one, you don't THINK.

"I have been on this forum for several years and have always been polite to everyone maybe it's time to move on I guess."

Please do, this is not a forum for lawbreakers and enablers. I suggest a CB forum where you will feel right at home among your fellow criminals.

"Since 10m is the only amateur band the attacker has HF phone privileges on, its no wonder he is fussy about CBrs encroaching on it."

That's neither here nor there, as a ham I defend our bands and privileges to operate regardless.

"You can't take anyone too seriously here, don't be afraid to share your thoughts or experiences."

Obviously he doesn't take very much seriously especially the law and the rights of others. As for me I DO take them QUITE seriously and make myself heard. Silence is tacit approval, I do NOT approve and will NOT be silenced.

"Just blow it off, no reason to leave."

No reason to leave provided he does not promote illegal activities, otherwise I have a door to show him. Maybe you haven't noticed I stand not only in defense of Amateur Radio but this forum as well. This is not a place to promote lawbreaking although some may disagree with me. Ask yourself "What would Lindsay think?" if you don't care what I think, you may change your mind about what you say.

"Been a good thread."

Yup, sure was up to a point, the point where the line was crossed.