Help! Rubby ducky antenna is better than a Diamond Discone??

[DracosIgnis]

First, let me take my foot out of my mouth since I recently reported that since I put up a Diamond discone that my reception was great.

I’ve got a 246T with a 25 foot LMR-400 jumper attached to a Diamond discone. I “thought” things were great until last night when just for fun I decided to monitor an adjacent city, about 20 miles or so.

After getting nothing from the discone, I assumed it was distance and I wasn’t going to be able to monitor it. As I picked up my handheld to go to another room, I attached the stock rubber ducky antenna, and to my surprise, it comes in fairly clear. Attach the discone again and waited… nothing. The system is an 800mhz Edacs.

So, I get a system 20 miles away with a stock antenna but not my nice base antenna. Any thoughts as to what I can check out? Is there a way to test an antenna? (as in the coax making a good connection?)

Mike

 

[N_Jay]

Re: Help! Rubby ducky antenna is better than a Diamond Disco

Any thoughts as to what I can check out? Is there a way to test an antenna? (as in the coax making a good connection?)

Mike

Bad antenna, or line,

OR,

Local interference.

 

[Al42]

Re: Help! Rubby ducky antenna is better than a Diamond Disco

So, I get a system 20 miles away with a stock antenna but not my nice base antenna. Any thoughts as to what I can check out? Is there a way to test an antenna? (as in the coax making a good connection?)

Mike

First, what frequency is the discone cut for? (How long are the bottom elements?) And what frequency is the system on?

Second:

The resistance from the center pin from one end to the other should be as low as possible (an ohm or two at most). Same with ground from one end to the other. Ground to center pin should be open - infinite resistance.

Connect the cable back to the antenna and resistance from center to ground should still be infinite.

But if the antenna isn't designed to receive the system's frequency good cable won't make it better. (A discone only receives up to 4 times the frequency at which the bottom elements are 1/4 wavelength, so a 30 MHz antenna is not so good at 150 or above.)

 

[DracosIgnis]

Thanks for the reply.

First:
The bottom elements are 32.3" according the spec sheet and I'm after a frequency of 868.000 - 869.000.

Second:
Everything you stated checks out. Inner 0.6, outer 0.7, ground to center is open.

Next, ground to center with antenna attached is open.

Finally:
By no means do I really understand "technically" how antennas work, so could you please explain why there is continuity from the inner coax to the "upper" portion of the discone but not to the lower portion and then by the same token, why there is continuity from ground to the "lower" portion of the discone but not the upper??

Thanks again!

Mike

 

[N_Jay]

Thanks for the reply.

First:
The bottom elements are 32.3" according the spec sheet and I'm after a frequency of 868.000 - 869.000.

Second:
Everything you stated checks out. Inner 0.6, outer 0.7, ground to center is open.

Next, ground to center with antenna attached is open.

Finally:
By no means do I really understand "technically" how antennas work, so could you please explain why there is continuity from the inner coax to the "upper" portion of the discone but not to the lower portion and then by the same token, why there is continuity from ground to the "lower" portion of the discone but not the upper??

Thanks again!

Mike

Well that antenna is only optimum from about 90 MHz to about 350 MHz.

Discones do work above their designed frequency but the gain falls off.
They work very poorly below the minimum.

(I'm still betting on a problem like a bad connector or interference) :wink:

 

[Al42]

Finally:
By no means do I really understand "technically" how antennas work, so could you please explain why there is continuity from the inner coax to the "upper" portion of the discone but not to the lower portion and then by the same token, why there is continuity from ground to the "lower" portion of the discone but not the upper??

Why, as in how did they do it, or as in why did they do it?

How they do it is simple - the center conductor is connected to the top part, the shield is connected to the bottom part.

Why they do it that way? Without going into a long (like weeks of study on your part) explanation - because it works.

 

[TeRayCodA]

Need more info:

What is it mounted to?
How high is it?
Is it near anything metal(roofing,siding,ect.)
LMR 400 coax connectors already "factory crimped"?
Is ALL the hardware tight-n-snug?
Antenna coax connector tight-n-snug?
What type of adapter to scanner are you using?

-sorry if some of these were already answered in your first post,just trying to help.

Also,if it is possible,take the coax,clip ohm meter to both connectors,outer(ground/shield)and "wiggle"the connector,observing the ohm meter.Same thing applies to the center conductor.

You're looking for a possible short.

 

[mastr]

A properly designed discone will work over a greater than 10 to 1 frequency range with reasonable gain. Most people who report poor gain on higher frequencies are "reporting" increased cable losses.

The discone's elements act as a transmission line transformer of sorts; for instance, an outgoing signal travels along the elements until the spacing thereof reaches a point that couples the signal into the air. Naturally, the reverse applies for incoming signals. FWIW, there is no reason other than ease of assembly/design that the "disc" is connected to the center conductor and shield to the "cone". Having the cone assembly grounded makes it easier (cheaper) to build.

For an example of a properly designed discone check out the Interad Ltd. product line; their model 5007 is (rated for 90-1200 mhz) very nicely constructed, and meets or exceeds the published specifications. However, they are a bit on the expensive side for personal use at over $3k.

 

[N_Jay]

A properly designed discone will work over a greater than 10 to 1 frequency range with reasonable gain.

Most of the information I have seen (from design guides, not sales liturature) indicates 3:1 or 4:1.

The roll off at the top is soft so you can easily extend the range by being more generious on how you define "reasonable gain".

 

[mastr]

Most of the information I have seen (from design guides, not sales liturature) indicates 3:1 or 4:1.

The roll off at the top is soft so you can easily extend the range by being more generious on how you define "reasonable gain".

That is true-gain figures don't mean as much in my application as the results (or lack thereof) when a distant unit calls our facility. Even the best discone won't out-perform a band-specific antenna, but 90 percent of the time, the difference won't be significant, given a reasonable antenna location.

 

[PluckyPleco]

Huh?

A properly designed discone will work over a greater than 10 to 1 frequency range with reasonable gain.

Gain? On a discone? Maybe a couple of dB over isotropic...

Could you clarify what you meant?

Plec

 

[PluckyPleco]

After getting nothing from the discone, I assumed it was distance and I wasn’t going to be able to monitor it. As I picked up my handheld to go to another room, I attached the stock rubber ducky antenna, and to my surprise, it comes in fairly clear. Attach the discone again and waited… nothing. The system is an 800mhz Edacs.

What sort of feed line are you using? How long of a run?

Plec

 

[kf4lne]

Gain? On a discone? Maybe a couple of dB over isotropic...

Could you clarify what you meant?

Plec

A discone has about 2.5db gain over isotropic, in reality they are unity gain antennas because isotropic it has teh same properties in all directions in space, and since your discone isn't installed in an empty universe with nothing else (including feedline) then that means your discone will have about 0db gain in reality if even that much. I think maybe a discone is actually lossy at a lot of frequencies. Never trust an isotropic gain claim, isotropic has no basis in reality because in reality everything surrounding the antenna will have an effect on its gain.

 

[N_Jay]

Gain? On a discone? Maybe a couple of dB over isotropic...

Could you clarify what you meant?

Plec

I am sure what is meant is "Without unreasonable loss"

Since antanna gain is measured relitive to some reference point, his statement is fine.

 

[N_Jay]

A discone has about 2.5db gain over isotropic, in reality they are unity gain antennas because isotropic it has teh same properties in all directions in space, and since your discone isn't installed in an empty universe with nothing else (including feedline) then that means your discone will have about 0db gain in reality if even that much. I think maybe a discone is actually lossy at a lot of frequencies. Never trust an isotropic gain claim, isotropic has no basis in reality because in reality everything surrounding the antenna will have an effect on its gain.

You are missunderstanding what dBi is.

It is just as relevent in any situation as any other gain measurment in dB.

It just has a different 0dB point.

 

[mastr]

Gain? On a discone? Maybe a couple of dB over isotropic...

Could you clarify what you meant?

Plec

I'll be glad to; first one must realize that "gain" can be a negative figure, and that there are valid reasons why negative gain can be accepted.

For instance, I have a $16.95 Hustler branded discone (that barely is one with only six total elements), that has been in service for over 15 years at my home. 99 percent of the time, it is used on an old Regency scanner to monitor public safety and "ham" repeaters within 25 miles or so. Once in a while over the years, I have pressed it into service for a control station on one of the repeaters or for connection to a service monitor. It has always performed "good enough" in that I could hear and be heard as needed. The VSWR was sometimes alarming, but never cost me any PA transistors. At $16.95 for an antenna that works from 140-860 mhz - just over a dollar a year. If it is 10 db down from what a band specific antenna would be, who cares? If I am able to hear and be heard over the desired range, that is "reasonable" to me.

At the other end of the scale, I recently tested a discone that cost more than 200 times that $16.95 figure. When fed with a length of Andrews LDF-4-50, it has a VSWR of less than 1.5-1 anywhere I could round up equipment to dump RF in to it. Further, it is constructed in a fashion that would make it a functional tool to defend one's self with if need be. It can be assembled from its package and in service in 10 minutes, and re-packed as quickly. The rugged construction and speed of deployment make up for the lower gain as compared to say, a DB-224. I can have this antenna on the roof of a building or mobile command truck and working before the db-224 is on its way up there. Once again that is "reasonable" for my application.

Fortunately, most of the users of "my" radio system are only concerned with successfully communicating, and not gain figures. It it works, that is the desired effect.

 

[MacombMonitor]

First, let me take my foot out of my mouth since I recently reported that since I put up a Diamond discone that my reception was great.

I’ve got a 246T with a 25 foot LMR-400 jumper attached to a Diamond discone. I “thought” things were great until last night when just for fun I decided to monitor an adjacent city, about 20 miles or so.

After getting nothing from the discone, I assumed it was distance and I wasn’t going to be able to monitor it. As I picked up my handheld to go to another room, I attached the stock rubber ducky antenna, and to my surprise, it comes in fairly clear. Attach the discone again and waited… nothing. The system is an 800mhz Edacs.

So, I get a system 20 miles away with a stock antenna but not my nice base antenna. Any thoughts as to what I can check out? Is there a way to test an antenna? (as in the coax making a good connection?)

Mike

This comes as no surprise. LOUMAAG pointed this observation out in a similar thread a couple weeks ago. In the 800MHz band, this has been my experience as well. I think most of the newer scanners are overloaded by outside antenna systems. I'm currently running all my scanners on their stock antennas, indoors, and I can clearly pick up most of what I want, without any problems.