General Antenna Arrangement??

[Roscoe1961]

I have a 40’-50’ tower beside my bungalow (that has a steel roof) and I want to put my antennas up Once. I have a homemade off centre dipole, I will be soon building a stainless steel discone, and I also want to put up a 9’ CB whip.
My thoughts are the whip can go on the top center. My steel roof will be 25’ below and to one side of this. The steel roof will be on the north side of the antennas and I will want to talk south east and south west. I’m not sure if this setup will work?
I was thinking of building cross braces to U bolt on the tower and putting the dipole on the south side about 5’ below the whip and the discone on the north side of the tower. I could easily make the dipole directional from east to south to west. With the steel tower on the north side of the dipole I don’t know how “ directional” the dipole will be?? The discone will be a complete experiment! Both the discone and the dipole will solely be for scanner listening.
Any thoughts would be appreciated

 

[jonwienke]

It's fairly common for discones to have a vertical attached to top center to add response in a band below the range of the discone.

 

[Roscoe1961]

It's fairly common for discones to have a vertical attached to top center to add response in a band below the range of the discone.

I’ve got an idea of the discone that I’ll build, I just need some more parts, dimensions, and time!!

 

[Roscoe1961]

Ok, I've got the dipole up and running, doing OK.
Now on to the Discone
I'd like to build this for general listening, nothing specific. I am out in the country, not close to too much at all. OPP, EMS, and Fire are in the area occasionally. My plans have changed a little since the post above, Todays idea is a Stainless Steel discone made with 1/8" X 12" horizontal radials, and 3/32" X 30" 45* bottom elements. The tips of the elements will be about 24" in a circular diameter. Everything will be tig welded, so nothing loose or rattling. On the top center I'd like to incorporate a 9' SS whip so I can transmit with a CB, both AM and SSB. I've had this whip and radio for many years on many different vehicles in many configurations and no problems with SWR. The difference this time would be the horizontal radials.

The question is should the radials be part of the top part of the discone, (Disc) hence connected to the center lead of the coax or should they be part of the bottom of the cone. if they are the bottom then no problem, if they go on the disc then i can see the swr could be an issue.

 

[prcguy]

You can't just stick a whip on a Discone, otherwise its no longer a Discone. Whips on a Discone are usually base loaded with a certain amount of base coil as an inductor so the whip is decoupled and somewhat invisible to the Discone within its operational frequency range. The whip with coil will then resonate at some frequency well below where the Discone works and CB is ok but you can't stick a full size 9ft whip on one.

If you just stick a 9ft whip on a Discone what will happen is the whip is not decoupled at higer freqs like 100MHz and above and it will resonate at many VHF and UHF frequencies on harmonics and will load down the impedance of the Discone at those frequencies in addition to the whip having very undesirable radiation pattern in the harmonic mode. A Discone is a aperture type antenna that launches a vertically polarized wave from the gap between the cone and disc and when you have a low frequency whip on top its working in a ground plane mode with a very insufficient ground plane. A VHF/UHF Discone with a properly designed whip on top for CB (if there is such a thing) will suck on CB compared to the same antenna on a larger ground plane like a car roof.

To answer your actual question, the cone is connected to the shield of the coax and the disc is connected to the coax center conductor. The cone should come to a point under the disc if possible and the distance between the top of the cone and the disc is critical for impedance matching. Usually you would build the cone and disc then connect coax and vary the gap between cone and disc while sweeping the entire frequency range and adjusting it for the best overall match. Then make whatever insulator is going to hold everything together keeping the proper gap. The cone shape should be an equilateral triangle.

Ok, I've got the dipole up and running, doing OK.
Now on to the Discone
I'd like to build this for general listening, nothing specific. I am out in the country, not close to too much at all. OPP, EMS, and Fire are in the area occasionally. My plans have changed a little since the post above, Todays idea is a Stainless Steel discone made with 1/8" X 12" horizontal radials, and 3/32" X 30" 45* bottom elements. The tips of the elements will be about 24" in a circular diameter. Everything will be tig welded, so nothing loose or rattling. On the top center I'd like to incorporate a 9' SS whip so I can transmit with a CB, both AM and SSB. I've had this whip and radio for many years on many different vehicles in many configurations and no problems with SWR. The difference this time would be the horizontal radials.

The question is should the radials be part of the top part of the discone, (Disc) hence connected to the center lead of the coax or should they be part of the bottom of the cone. if they are the bottom then no problem, if they go on the disc then i can see the swr could be an issue.

 

[Ubbe]

Your dimensions suggest that you would like to receive the FM broadcast band and upwards in frequency. You usually can receive up to 7 times the lowest frequency, in your case 700MHz, then the signal deteriorates above that.

Make the discone work from 118MHz and upwards as FM broadcast are usually strong enough to be received with any antenna and usually are too strong and needs a FM trap filter to not make a scanner overload and loose sensitivity. The lower cone radials should be 1/4 wave lenght of the lowest frequency and the disc diameter 70% of that. And as pointed out the angle are 60* for the bottom cone elements, so the antenna gets more narrow than with a 45* angle. The element lenghts for 118MHz are 24" and 8"

Discone antenna - Wikipedia

en.wikipedia.org

/Ubbe