Discone sweep

[mmckenna]

More playing with the work toys….

I brought my good spectrum analyzer with tracking generator home from work to test the antenna I just installed on my wife's truck. I swept the VHF quarter wave on her truck to confirm is was good, and for kicks, I swept it for UHF. That's posted in another thread. 1/4 wave VHF antenna on UHF

For giggles, I hooked it up to the discone I have up on the roof of my home. If I recall correctly, this is the Diamond D130NJ Diamond® Antenna ~ D130NJ Super Discone Antenna
I scored this antenna for free about 12 years ago. I put it up on the roof as a temporary VHF antenna, at least until I got a better antenna. But then I sort of forgot about it. It did what I needed it to do and just never got around to replacing it.

It has all it's radials and does have the low band whip sitting on top. It's mounted on a pole off the side of the house, above the roof line by about 10 feet and clear of all obstructions. Feed line is Times Microwave LMR-600, probably about 35 feet worth.

Here's the 30MHz to 1.2GHz sweep. Markers are at useful points, like the 2 meter band, 70cm, etc. This is looking at SWR, so take it with a grain of salt if you are just using it for receive:


I walked the first marker down to the 50MHz point to see what it looked like there. Not stellar, but not offensive:

Again, that's with the low band whip sitting on top, so some action on the 6 meter band.

And for giggles, I stretched it down to 25MHz, just to see what it would look like as a CB antenna (poor):


Again, not trying to sell you on anything, just some screen shots for those of us that learn visually.

 

[mmckenna]

You can see it's got some nasty performance down just under the 2 meter band. Without the top whip, this thing probably sucks eggs much below 2 meters.

 

[K4EET]

Thanks @mmckenna for this post. I've never (until recently) had a discone to sweep and I actually didn't think it would even look that good. I am sort of impressed. A friend of mine in another state recently sent me a brand new Tram Model 1410 discone still in the original packaging and shipping container that they were not using. I no longer have direct access to good test equipment like you have but would love to see a comparison if somebody out there in cyberspace can make one. If you are near me with the test equipment, you can even borrow my discone to do the sweeps. I realize that the Tram will probably not last as long as your Diamond @mmckenna. But again, thanks for the post! 73, Dave K4EET

 

[n3obl]

@mmckenna what are you using to sweep the antenna? Would you mind posting a pic?

 

[mmckenna]

@mmckenna what are you using to sweep the antenna? Would you mind posting a pic?

Agilent/Keysight 9342c 7GHz spectrum analyzer with tracking generator and modulation analysis options. Was a short term solution before I could procure a full service monitor. Now it's mainly used for hunting interference and sweeping antennas.

https://www.keysight.com/us/en/assets/7018-02985/brochures/5990-8024.pdf

Also using a HP directional coupler to sweep the antennas. Inject the tracking generator output into one port, connect the antenna under test to the other, and reflected power goes into the spectrum analyzer input.

 

[mmckenna]

If you are near me with the test equipment, you can even borrow my discone to do the sweeps.

Unfortunately I'm on the wrong side of the continent for that, but I'd do it if I could.

 

[K4EET]

This thread should be a "sticky" or possibly made part of a broader Reference Wiki to collect data like this when it becomes available.

 

[spongella]

Thanks OM, great pics and info. I have the same exact antenna mounted on the roof, originally installed for receiving only. The SWR on the 2m band is >2:1 so I guess the whip should be adjusted. Feeding it with RG/6 so there's a slight mismatch. Will take my MFJ antenna analyzer with me up to the roof when I do adjust the whip. Great RX antenna though.

Your discone's SWR looks pretty low at 149.5 megs. Wonder how it is on the 2m ham band though.

Those spec analyzers with TG option are great. Unfortunately I had one and sold it. What was I thinking? Am now using an old HP 8591A, but no TG installed.

Thanks again.

 

[mmckenna]

Thanks OM, great pics and info. I have the same exact antenna mounted on the roof, originally installed for receiving only. The SWR on the 2m band is >2:1 so I guess the whip should be adjusted. Feeding it with RG/6 so there's a slight mismatch. Will take my MFJ antenna analyzer with me up to the roof when I do adjust the whip.

The whip on top is for low band. Not sure if/how it would impact 2 meter cover, but won't hurt to try. Let us know how it goes.

 

[nd5y]

Remember that since a spectrum analyzer & tracking generator can't measure radiation patterns it can't tell you how good an antenna really is. You can have low SWR and a useless radiation pattern. If you sweep a 50 ohm resistor it will show a nice low SWR everywhere.

 

[mmckenna]

Remember that since a spectrum analyzer & tracking generator can't measure radiation patterns it can't tell you how good an antenna really is. You can have low SWR and a useless radiation pattern. If you sweep a 50 ohm resistor it will show a nice low SWR everywhere.

Yes, exactly. And precisely why I added the line: "Again, not trying to sell you on anything, just some screen shots for those of us that learn visually."

 

[mmckenna]

If you sweep a 50 ohm resistor it will show a nice low SWR everywhere.

I did!



Decibel 50Ω 100watt load.

I think I'll sell these on e-Bay as 'wide band' antennas to the Baofeng crowd.

 

[cmdrwill]

You can have low SWR and a useless radiation pattern.

And the "useless radiation pattern" and or "low SWR",does NOT make a good antenna.

 

[Ubbe]

The whip on top is for low band. Not sure if/how it would impact 2 meter cover, but won't hurt to try. Let us know how it goes.

A vertical whip usually works fine at 3x it's frequency, so that a 50Mhz whip might have low SWR at 150MHz. Radio amateurs frequency bands are usually made with 3x intervals to be able to use the same antenna at different bands, like 2meters on 70cm I.E. 145MHz to 435MHz but for some reason US doesn't use the same amateur frequencies as the rest of the world.

There's a loading coil at the bottom of the whip that should go high impedance at higher frequencies to stop it from interacting with the discones frequency range, but could still have some impact.

/Ubbe

 

[videobruce]

Questions;
What model Agilent SA is that?
The 'scale' on the left, it's that in terms of a SWR ratio?
Can you put that back in dBmV or at least dBm?

I've never have seen anything like that before from a SA and I have had probably a dozen over the years I have had experience with (only half as many actually owned )

 

[mmckenna]

Questions;
What model Agilent SA is that?

Agilent N9342C

The 'scale' on the left, it's that in terms of a SWR ratio?

Yes, it's SWR.

Can you put that back in dBmV or at least dBm?

Yes, you can choose to display as return loss or SWR. I use SWR for stuff like this since it's easier for most hobbyists to understand.

 

[videobruce]

It's just that I'm so use to looking at dBmV, the 'SWR' is almost 'foreign'.

Ok, what would you say of this for a scan of another discone?

 

[merlin]

Yea, I swept my discone and got pretty much the same results. The dips seem to be where the resonance is.
For the heck, I connected an antenna to the generator, the discone into the analyzer and had some strong signals around 95 and 200 Mhz. The humps started falling off above 900 Mhz, but quite surprised at the levels I got.
Very decent antenna from 50 to 950 Mhz, no gain but a good amp about 15 db will solve that.
My analyzer is the HP 8590, ,1986 version, the generator 8444A-option 059. Oldies but goodies.
No TG though.

 

[merlin]

It's just that I'm so use to looking at dBmV, the 'SWR' is almost 'foreign'.

Ok, what would you say of this for a scan of another discone?

View attachment 100007

Very close to what I got in that spectrum, not too bad.

 

[E5911]

What is the difference between these: Wide Band Discone Antenna - Antenna Experts
and other discones