UHF (SO-239) antenna bases?

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nsinbc
Posts: 2
Joined: Fri Mar 12, 2021 1:04 am

UHF (SO-239) antenna bases?

Post by nsinbc » Sat Mar 20, 2021 5:53 am

Fairly new to this and hoping some kind radio nerd will put me out of my misery...

Here's my dilemma:

I bought the Diamond D220R Discone antenna for my apartment balcony. Very excited to try this out.

It terminates in a male UHF connector and doesn't come with a mount. The options on my balcony seem limited to:
  • Buy a mag mount like this one and stick it onto some heavy metal object, maybe even the railing (though perhaps the railing would cause some weirdness?)
  • Maybe a clamp mount of some kind? But unfortunately I can't seem to find any UHF clamp mounts that aren't for a vehicle.
The reason I'm not super thrilled about option #1 is that enormously long coax cable (15m), which I definitely do not need for my application. ALL of the mag mounts like this one I've seen seem to have long cables. From the reading I've done, it seems attenuation and interference are a real problem with longer cables. I'd like to avoid that if I can.

Does anyone know of a clamp or magnetic style mount with a female UHF connector that either has a short cable or just a place to connect your own?

Definitely open to other ideas and feedback, too!

joe36
Posts: 53
Joined: Wed Jul 20, 2016 11:09 pm
Location: Cass co. in. 46994

Re: UHF (SO-239) antenna bases?

Post by joe36 » Tue Mar 23, 2021 2:28 pm

You DO NOT WANT! The magnetic mount. It will not work for this application.
Do you have a railing with vertical supports? If so you might be able to use hose clamps to hold it on.
The Discone antenna is a very good choice for a wide frequency range. The type of antenna you have purchased. The diamond D220 R. Is a modified discone design. The vertical antenna portion above the horizontal plane is not part of a discone design.
The vertical antenna does serve a purpose. It is a tuned vertical antenna element worked against a ground plane. The coil in the element would indicate it is designed tor 2 separate primary frequencies.
The discone antenna has 0db gain over a very wide frequency range.
It is a very good choice in that it has a very wide frequency range with consistent gain.
A dipole antenna has 3db gain in free space over a narrow frequency range. Outside of this frequency range the impedance mismatch makes the antenna inefficient.
Have you considered a lighting stand as a antenna mount?
Joe

snn47
Posts: 253
Joined: Tue Dec 27, 2016 11:00 pm

Re: UHF (SO-239) antenna bases?

Post by snn47 » Fri Mar 26, 2021 9:23 pm

There are a lot of incorrect statements in this reply, the most important is
A dipole antenna has 3db gain in free space over a narrow frequency range. Outside of this frequency range the impedance mismatch makes the antenna inefficient.
Antenna efficiency meaning how much power of a signal is received and converted into a signal for your receiver is not the same as impedance match.

There is no impedance missmatch, if you connect a shielded and therefore non radiating termination with the correct impedance instead of an antenna But even though there is no missmatch, you cannot receive a thing since you did not connect an antenna, but a shielded termination.

As long as you have some form of conductor connected to the transmission line and do not short it, you will receive signals even well outside the frequency range it is supposed to work and despite impedance missmatch, e.g. I did receive a time signal on 0.0775 MHz with just 20 m of wire, even though a 1/4 wavelength is 1000 m

How well you will receive signals will depend on many factors which would take to long to write down in a few pages, e.g. are the signals horizontally or vertically polarized, what is the gain of the antenna pattern at the transmitt frequency towards the signal source and what is the EIRP transmitted towards you.

Antenna mount are easy to built, e.g. a long flat piece of metal, with a whole drilled into one end and either a UHF PL-259 Male to SO-239 Female 4 hole Panel Mount or a SO-239 UHF Female To SO239 Connector Bulkhead With Nut for the antenna. Drill wholes on the other end for mounting it on the suporting surface and bend it into shape.

joe36
Posts: 53
Joined: Wed Jul 20, 2016 11:09 pm
Location: Cass co. in. 46994

Re: UHF (SO-239) antenna bases?

Post by joe36 » Sat Mar 27, 2021 7:21 am

I think you misunderstood my post. Or I misunderstood yours?


“Antenna efficiency meaning how much power of a signal is received and converted into a signal for your receiver is not the same as impedance match”.

At 50Mhz a whip style antenna 82" long worked against ground has a radiation resistance of 50 ohms. If this same antenna was say 12" it would have a radiation resistance of 1 ohm.
A 12 inch wire will receive a signal! The front end mismatch along with the fact that such a short fraction of a wavelength not being efficient will seriously degrade the signal strength.
You received a signal at .0775 MHZ with a 20 meter wire. You had a radiation resistance of .1 ohm. This obviously worked for you. It would have worked much better if you could have gotten the resistance up by using a wire closer to resonance
This would have given you a larger capture area along with an input impedance closer to that required.



You sort of lost me here?

“There is no impedance mismatch, if you connect a shielded and therefore non radiating termination with the correct impedance instead of an antenna But even though there is no mismatch, you cannot receive a thing since you did not connect an antenna, but a shielded termination”.

It is way past my bed time but I must admit. I reed your post several times. I do not understand how this statement comes into play.
A resistive termination “dummy load” provides a shielded Impedance match to ground. It is intended to dissipate power from the antenna to ground. This ensure there is no signal present on the antenna.

Joe

snn47
Posts: 253
Joined: Tue Dec 27, 2016 11:00 pm

Re: UHF (SO-239) antenna bases?

Post by snn47 » Sat Mar 27, 2021 9:09 pm

Any mismatch between the receiver, the transmission line and associated components (e.g. connect-or, switches) and the antenna will have some impact on how much the signal from the antenna is attenuated, before it is at the receiver antenna input., but it does not determine if a antenna is providing you with good reception .

Even very short vertical antenna with a top capacitance and good ground plane, will provide similarly good reception (~4.8 dBi) as a ¼ lambda antenna. The drawback is the low impedance, which will get lower the lower the operating frequency is, in average <<1 Ohm.
The antenna efficiency for transmit and receive is, despite being very short, is still similar to that of ¼ Lambda ground plane. But such short antenna requires matching of the low antenna impedance.

Again my example of connecting a non radiating termination instead of an antenna, you will have no mismatch to worry about but will not receive much since the antenna efficiency for reception is 0%.

Antenna pattern gain and directivity: The antenna pattern and gain towards the signal source is important. The main beam directivity of the azimuth and elevation toward the signal source is important.
The optimal elevation of the main beam depends on how high the signal source is (reflected) above ground. For SW reception via reflections at the atmosphere, or VHF or higher frequency reception e.g. of aircraft or satellites, a main beam close to the horizon will produce only weak signals, The same applies in azimuth if the main gain is not towards the signal source.
Unfortunately The antenna azimuth and elevation of the antenna lobes will vary in gain, azimuth and elevation with frequency, even for broad and or broadband antenna designs
Also the higher the gain will increase above the gain of an isotropic antenna the more directive and the narrower the azimuth and/or elevation beamwidth of an antenna will become.

So a lower gain antenna might serve SWL better than trying to increase gain at the cost of not beeing able to receive all possible sources, unless you need a high gain to receive a sources you cannot receive with low gain..
At 50Mhz a whip style antenna 82" long worked against ground has a radiation resistance of 50 ohms. If this same antenna was say 12" it would have a radiation resistance of 1 ohm.
A 12 inch wire will receive a signal! The front end mismatch along with the fact that such a short fraction of a wavelength not being efficient will seriously degrade the signal strength.
Do you mean a whip of 1/4 Lambda length for 50 MHz?
A 82 " (~2.08m) whip, without accounting for thickness and the corresponding effects that require it to be shortened somewhat, would be resonant at 36 MHz.
For 50 MHz a 1/4 wavelength antenna would be ~1.5 m (59 "),
A 1/4 Lambda above a ground plane/counterpoise requires it to have a groundplane/counterpoise to form a ground plane. The impedance is just about ~36 Ohm, or half of the impedance of a 1/2 Lambda Dipole.

A dipole antenna has 3db gain in free space over a narrow frequency range. Outside of this fre-quency range the impedance mismatch makes the antenna inefficient.
Did you mean Fullwave or ½ wave dipole?
A fullwave dipole has ~3.8 dBi, a ½ dipole ~2.15 dBi, and ¼ groundplane ~5.2 dBi gain.
With increased diameter of the antenna element in reference to the wavelength the bandwidth will increase, so only thin dipoles will be narrowband.
If you further increase the thickness by widening the antenna elements toward the end, you’ll get a wideband dipole (Bi-cone), which is a very broadband dipole.
You received a signal at .0775 MHZ with a 20 meter wire. You had a radiation resistance of .1 ohm. This obviously worked for you. It would have worked much better if you could have gotten the resistance up by using a wire closer to resonance
This would have given you a larger capture area along with an input impedance closer to that required.
The point I was trying to make was, since we want to receive only, we do not have to worry about matching the antenna to the transmitter to avoid missmatch. Unless we are interested only in very narrow band-segements, there are ver few options to optimize a broadband reception.
So we have to and IMHO can live with in relation to the wavelength very small/short antennas like e.g. the 20 m wire for a wavelength of 3871 m. I don't know about you, but I am limited to about 40m in horizonal length for antenna where I live today. 20m of cooper wire allowed me not onlay recepetion down to 77.7 kHz but also to transmitt on most shortwave bands between 80 m and 10 m available to hams. It is a compromise, but what in life is optimal?

joe36
Posts: 53
Joined: Wed Jul 20, 2016 11:09 pm
Location: Cass co. in. 46994

Re: UHF (SO-239) antenna bases?

Post by joe36 » Sat Mar 27, 2021 11:23 pm

A dipole ½ wavelength has around 3db over A 1/4 wave monopole worked against a ½ wave diameter conductive disk. This is due to the radiation pattern of a dipole being a figure 8 and a 1/4 worked against a conductive surface. This gives no account to conductor diameter or velocity factor. Something that is too in-depth for this post
You have given a lot of good information but I am still confused as to why you keep comparing a mismatch to a shielded load? The math for a mismatch % of power transfer is a set formula. It at no time equals zero power transfer. A mismatched paperclip will transfer energy anyplace in the spectrum. It will be highly unlikely to be strong enough to be detected unless the exciter signal is very strong.
As you mention any antenna is a compromise and we work within limitations.
It depends a lot on your equipment. I have had ham radios that received several strong CW signals without the antenna attached. That was a sensitive receiver and a strong signal.
With a good receiver you can use just about anything to receive but with the little $20 RTL The dynamic range is so low that it requires a bit better antenna.
I am curious as to what type of equipment you are running? I have been playing with a Hack RF1 lately. The dynamic range is pitiful.
We just moved 3 days ago to this location. I could probably put up a 100 meter length of wire. For now all I have up is 25 meter total length dipole at around 10 meters.
I have a discone with 1 meter elements yet to be moved. The plans after that will be to build 2 new discones one with 45cm elements and one with a 10 cm copper cone and a vlf antenna.
The discone is a very wide band antenna that is under appreciated.
Beyond that antennas are like mushrooms. They just seem to pop up.
Oh what a tangled web we weave when we practice to receive!

Joe

SWL452
Posts: 5
Joined: Sat May 22, 2021 4:41 am

Re: UHF (SO-239) antenna bases?

Post by SWL452 » Sat May 22, 2021 5:32 am

This is my solution. Either a small tripod antenna mount:
https://www.ebay.com/itm/221645013632
or clamps that can accommodate up to a 2 inch mast:
https://www.ebay.com/itm/390742727652
To that, you can fix a SO239 mount that will take the antenna:
https://www.amazon.com/Tram-Stainless-S ... B08F9V4FDV
This mount can bolt to the top leg bolt of the tripod, or clamp to a support vertical of the railing.
Now my discone fixes to a 1 1/4" OD mast with a female 'N' connector.
The coax runs down inside the 5 foot mast. That supported with the tripod.
Discones are not dipoles or ground planes and their broadband range, they have less than unity gain.
They work OK but I do suggest a preamp.
My choice, Scientific Atlanta drop amp W/15Db gain
https://www.ebay.com/itm/114191913810?h ... Swtkhem4Gs
This setup will give good performance from 60Mhz to 1.3Ghz

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