Altering a discone?

[SIMON11]

I have a discone antenna which I would like to improve if possible. I wasn't quite happy with the performance of my discone in the 2m and 70cm bands. I bought a diamond antenna specifically for these two bands quoting a gain of 4.5db and 7.5db respectively.
As discones don't seem to quote gain and if they do it is only 0.5db or very little I was expecting a vast improvement. Unfortunately the discone outperforms this antenna on both bands.
I am now just messing around with the discone to try and improve it's performance. I have doubled the length of the base legs and this seems to have improved the low frequencies.
The way I am thinking is that I do not need anything over about 470mhz. I would imagine that the short antenna near the top of the discone are for the higher frequencies. I am wondering if I alter the length of these if I can improve the 440mhz band as this is what I mainly listen to. If so what length would I need to make these for ultimate reception on this bandwidth? I am only experimenting I don't even know if this is feasible
Anybody know?

 

[SIMON11]

I have been messing about with my discone and I can take all the short antenna near the top off and it makes no difference. Extending them does nothing either. I can only think that each seperate antenna area is filtered for that particular band so that altering them for another frequency has no effect. Is this right?

 

[MacombMonitor]

Most discones, out of the box, are tuned for the 2m, and 70cm bands as good as they're going to get. That is why they usually say the antenna is appropriate for transmitting on those bands. So it's unlikely you'll be able to improve on the original design. Discones by their design are wide-band, but not high-gain. The shorter elements you refer to, (probably a little over 3"), are for the 800MHz band.

Bill

 

[Tweekerbob]

Simon,

The discone is a crappy antenna out of the box. What really troubles me is that you are under the impression based on your experience that the discone is outperforming the Diamond dual bander you bought??? There is something wrong with that picture. If you replaced the discone at the exact same spot with the Diamond, then you should notice a slight increase in signal. Keep in mind 4.5 dBi gain is NOT day and night difference. But a marginal signal heard on the discone SHOULD (possibly) sound better. There are a whole lot of factors, but in general, you should get better performance with the dual bander if you are listening to frequencies in its specified range.

I would check you coax, connectors, etc. just to be on the safe side.
\

Now for the discone; it is a disc and a cone. The ratio of the size of the disc and the size/shape of the cone must be kept the same. By lengthing the "Cone" section without adjusting the Disc section, you are basically throwing the antenna out of whack. For transmitting purposes, this is definitely NOT the right thing to do. But if you feel it gives you better reception performance, then experiment on... You will not hurt your receiver. The shorter elements again are the disc or capicitance hat as some will call it. It is not for 800 MHz. The overall size of the antenna will dictate the lowest resonant frequency, which in turn will dictate the highest resonant frequency, but the elements of a discone are not "tuned" for each band, they are simply pieces of the equation.

If you want to keep on experimenting here is the formula for the dimensions of a discone:

L = 2953/F

D = 2008/F

D = diameter of disk inches

L = side of cone inches

F = lowest frequency MHz

It might be fun to measure your unaltered discone and see what its lowest frequency is.

 

[kb2vxa]

Hi Simon and all,

Houston, we have a problem. There's obviously something wrong with the Diamond if it doesn't outperform a discone as one has unity gain. You don't mention the model but some require assembly of the vertical radiator and that's where things can go wrong. There's a ferrule that connects the upper and lower portions of the radiator so if it's not secure it makes poor contact or none at all and that's where the gain comes in, it's a phased colinear array. Then you have to be careful with the foam spacers or you can deform the delay lines when you slide it all together. Like most manufacturers they rate gain in Dbi so subtract 2.8 to get Dbd or true gain. Still it should perform well, mine does and it works like Gang Busters on the scanner too. Never mind 800MHz, that's another thread and another antenna.

You don't mention what brand discone you have but you described the typical hybrid. Aside from tuning the vertical section for 6M or VHF Lo Band whichever you prefer, always remember RULE NUMBER ONE, if it ain't broke, don't fix it. You can't improve on the design the engineers came up with, a discone is a discone and they haven't changed since the 40s when the Navy first used them. They understand a good thing, you can't improve on perfection. I think I read that in one of the NavPers radio training manuals somewhere.

"It might be fun to measure your unaltered discone and see what its lowest frequency is."
Been there, done that with an MFJ analyzer. Readings are all over the board, resonance occurs on 144 and 440MHz aproximately and the ground plane (I said it's a hybrid) resonates where you tune it somewhere between 30 and 60MHz typically. No matter, load impedance and SWR matter not for receiving, it's the overall bandwidth that matters. It resonates where it should so it's aproximately 50 ohms with an average 1.5:1 SWR on the Amateur bands. Depending on the manufacturer "your milage may vary" but not by much.

One final note, throw away the book and measure it. Having worked with electrical engineers in R&D I can't count all the times it has worked on paper and smoked on the bench. Then after I got the prototype working they asked how I did it and all I could say was "Uh, ah dunno, just twiddled this thang hyar and it wurked." That baffled the hell out of them and more than once I came in on Saturday to use the lab for one of my pet projects and there in the office was the CE fuming over his computer and babbling in Greek. No biggie, I just went over to the diner for coffee and a slice of pi.

 

[SIMON11]

Thanks for all your input I find it very interesting. I have no test equipment to check anything technically. I wouldn't know how to use it anyway. All I use is the meter in my scanner.
My antenna is a one piece diamond x50. All you do is screw 4 little rods in the bottom. They state it needs no setting up.
I have a regular ham for instance on 147.7mhz. When connected to the diamond I usually get a signal but with no indicators on my meter. When I connect the discone I get one bar. Since I have extended the legs and stuck bits on the upper fins ad lib I am getting two bars most of the time. I have also noticed since I did this that a frequency I listen to at 453.6 has now an extra bar on the metre. Only time will tell if this is due to what I have done or an atmosphere change.
I know that experts have been working on improving the discone for years. But my way of thinking was that they are working on it as a wideband antenna and they are usually concerned about size and weight. I thought that if I make everything bigger and concentrate on the 2m and 70cm bands that I could possibly improve it. My discone usually sits just over a metre and a half high. It is now well over 2m high as I doubled the length of the legs. I didn't realise that if I change the antenna length that I have to alter the cone size. I have no way of doing this so I can only experiment . I was wondering what would happen if I make the antenna legs thicker for instance. This discone is in my loft so I have plenty of room to work. I can't put it outside as it will no doubt just blow away where I live.
My theory came from when I was laid in bed listening on the rubber duck when I first got the scanner. I was listening to a low frequency with only one bar on the metre. I accidently touched the duck against the metal frame of the bed end and the meter shot up to full with a really clear signal. That made me think big was good.
Perhaps I should put my bed in the loft and fasten it to the coax.(Ha, Ha Ha). I do not at the moment understand the technicalities of antenna but hope to gain this knowledge eventually. Then I will be able to build a monster from scratch.


PS. I removed the shorter elements at the top and it made no difference to the reception that I could notice so I thought they must be for the upper frequencies?

 

[Tweekerbob]

My apoligies, Simon...I failed to realise (realize) you were from the U.K. oops...I gave you the formula for inches (you're on your own for conversions though ).

Again, as I think???, Warren pointed out, you may want to check the Diamond. If I am correct though, isn't that a one piece fiberglass radiator?? May not be much "checking" involved. In the realm of "scanner" antennas, "outperform" may be a relative reference. However, as far as theory is involved (and assuming all other variables begin equal), the diamond should "outperform" the discone. As Warren mentioned, I also have a dual bander (Cushcraft) that would melt the pants off of a discone (or burn the COTTON pants off of if one was not wearing plastic pants).

The discone is an interesting antenna, I feel; just not a very efficient one. If you want to experiment with them, particulary with the one you have already, by all means, do! Although I am one that will say that math ALWAYS has the explanation, I also contend that hey, if it works...roll with it (regardless of what the math says).

The shorter horizontal elements have nothing to do with the 800MHz band. Please see the formula I included in the previous post...the diameter of the "disc" is a simple function of the length of the cone or vice versa. It is the cone AND the disc (and the ratio of the two) that are both tuned to a specific frequency.

Simon, as a caveat to all, make sure you are not expecting TOO much out of your antenna(s). If you are looking to receive signals from beyond visual range, in most cases, you are asking too much of your antenna.

Happy Experimenting!

 

[SIMON11]

My apoligies, Simon...I failed to realise (realize) you were from the U.K. oops...I gave you the formula for inches (you're on your own for conversions though ).

Again, as I think???, Warren pointed out, you may want to check the Diamond. If I am correct though, isn't that a one piece fiberglass radiator?? May not be much "checking" involved. In the realm of "scanner" antennas, "outperform" may be a relative reference. However, as far as theory is involved (and assuming all other variables begin equal), the diamond should "outperform" the discone. As Warren mentioned, I also have a dual bander (Cushcraft) that would melt the pants off of a discone (or burn the COTTON pants off of if one was not wearing plastic pants).

The discone is an interesting antenna, I feel; just not a very efficient one. If you want to experiment with them, particulary with the one you have already, by all means, do! Although I am one that will say that math ALWAYS has the explanation, I also contend that hey, if it works...roll with it (regardless of what the math says).

The shorter horizontal elements have nothing to do with the 800MHz band. Please see the formula I included in the previous post...the diameter of the "disc" is a simple function of the length of the cone or vice versa. It is the cone AND the disc (and the ratio of the two) that are both tuned to a specific frequency.

Simon, as a caveat to all, make sure you are not expecting TOO much out of your antenna(s). If you are looking to receive signals from beyond visual range, in most cases, you are asking too much of your antenna.

Happy Experimenting!

Thanks for this info. I am one of the old school so I was brougt up with feet and inches, no problem.
Yes! My diamond is one piece. I was advised to get a cushcraft antenna but as it was twice the price of the diamond, I went for the latter. I wasn't convinced that it would be better and didn't want to spend over £100 on it. I got the diamond for half that.
I do live just over the side of a hill from the main city. Unfortunately most of the frequencies I like listening to come from here so I was trying to just increase the signal a bit. I also live near a harbour and a couple of ports. I can see lots of ships on the horizon about 10 miles away with a clear view to my antenna but I only here the ones entering the harbour or very close. Perhaps they don't transmit much once at sea?
I suppose I am disappointed in the diamond, it is over two metres long and only just fits in the loft. It is also twice the price of the discone I bought. I may give the supplier a ring and see what they say. They may take it back or exchange it for an even bigger one with more gain. I wonder if it could be faulty?
The signal furthest away that I am receiving and can clarify. Is a taxi firm from accross the bay about 8 miles.
I think as the weather gets better I will probably mount it on the chimney outside. This will give me that bit of extra signal that I want.
To be quite honest I just bought a Watson W889 telegainer which is 41cm long and if I go out in the garden it outperforms my diamond in the loft and almost equals the discone for peformance on these two bands. Now that's something to think about! I mounted the diamond temporarily on a twenty foot mast just to try it outside and it wasn't that much better than this.
This W889 IS the best value and performance antenna I have bought up to now and cost me less than £20.00 including postage. Well pleased with this!

 

[Tweekerbob]

Well, that is something. Although it is fairly unlikely, it is possible that the antenna is faulty; hard to tell what's going on inside the fiberglass. It certainly wouldn't be my first guess, but not outside the realm of possibility.

Again, if not done already, check your connectors and coax. Check the coax for continuity of both the center conductor and the braid, and also ensure that there is not a short between center conductor and braid. Secondly, just make sure that the connectors are snug and that they are making proper (solid) contact. Also, what kind of coax are you using and how many feet and how OLD is it???

Most of the time the problem is not with the antenna, but somewhere in the feedline system. The Diamond should be a fairly good performer when you put it up outside, as high as you can get it (keeping structural integrity in mind) and with good low loss coax and connectors.

I don't know much about ships (but I would guess that most of the radio traffic is going to be closer in to port), I live about 90 miles from the coast

Your telegainer is worth keeping around for the handheld. Just remember to "tune" it for the particular band you are listening to in order to maximise reception. This isn't the handiest formula for memorization, but 7132/frequency in MHz will yield how many cm's the antenna needs to be extended to. For 146 MHz 7132/146=48.9 cm.

Let us know more about your feedline and we can help you determine if it's that or the antenna.

 

[SIMON11]

Well, that is something. Although it is fairly unlikely, it is possible that the antenna is faulty; hard to tell what's going on inside the fiberglass. It certainly wouldn't be my first guess, but not outside the realm of possibility.

Again, if not done already, check your connectors and coax. Check the coax for continuity of both the center conductor and the braid, and also ensure that there is not a short between center conductor and braid. Secondly, just make sure that the connectors are snug and that they are making proper (solid) contact. Also, what kind of coax are you using and how many feet and how OLD is it???

Most of the time the problem is not with the antenna, but somewhere in the feedline system. The Diamond should be a fairly good performer when you put it up outside, as high as you can get it (keeping structural integrity in mind) and with good low loss coax and connectors.

I don't know much about ships (but I would guess that most of the radio traffic is going to be closer in to port), I live about 90 miles from the coast

Your telegainer is worth keeping around for the handheld. Just remember to "tune" it for the particular band you are listening to in order to maximise reception. This isn't the handiest formula for memorization, but 7132/frequency in MHz will yield how many cm's the antenna needs to be extended to. For 146 MHz 7132/146=48.9 cm.

Let us know more about your feedline and we can help you determine if it's that or the antenna.

I should put one thing straight here for the sake of diamonds reputation. This antenna is actually a diamond X50 copy with the same name. I bought it here.

http://www.users.zetnet.co.uk/knightscb/shop/index.html?target=dept_84.html&lang=en-gb

They state it performs exactly the same but maybe not? I contacted the firm and they say that they have never had a faulty one. They also say that the only way to test it is with a transceiver to test the SWR. As I only have a scanner I can't do that.

I have soldered the connections and checked the continuity. I have made a short lead only about 2 metres long so their is virtually no coax loss.
I am wondering now if I should have bought the cushcraft instead. But obviously I don't want to do that now in case I just waste another £100.00 if it dosen't perform any better.
The X50 is actually on better quality coax than my discone. RG58A/U - RG58 respectively.
I think now I will mount it outside on the chimney when the weather is warmer. This may just allow the extra bit of signal that I want. Although my discone seems to be better since I changed the antenna lengths.
Thanks for the info on my Watson. I will keep this in mind when using it. I have just been fully extending it up until now.