Design and wiring for a UHF antenna

[Jason Holland]

Hi,

Forgive my ignorance, but I am new to the concepts of antenna circuitry and design. I am trying to experiment with homemade UHF antennas for wireless microphone applications (470-698 MHz). I have read a great deal about antenna theory, and have some basic understanding of antenna design, but a few concepts are still unclear. Anyone who has an answer or idea, your help would be greatly appreciated:

1) How would one design a highly directional UHF antenna?

2) When designing a monopole antenna, is the coax connected directly to the radiating element? If one lead is connected to the radiating element, and the other is connected to a ground plane, wouldn't that create an open circuit?

3) How do you "balance" an antenna? I am confused as to how to measure the resistance/impedance of a radiating element, and how to balance a dipole antenna. (I do understand how to measure VSWR)

Thank you in advance for your help! I apologize if these are infantile questions, but I am just a rookie.

-Jason

 

[davenn]

Hi Jason
Welcome

1) How would one design a highly directional UHF antenna?

a highly directional antenna for 500 MHZ ( mid your range) is VERY big

2) When designing a monopole antenna, is the coax connected directly to the radiating element? If one lead is connected to the radiating element, and the other is connected to a ground plane, wouldn't that create an open circuit?

That's the way it should be connected ( centre of the coax to the base of the radiator, screen of the coax to the ground plane)

only to a DC current

3) How do you "balance" an antenna? I am confused as to how to measure the resistance/impedance of a radiating element, and how to balance a dipole antenna. (I do understand how to measure VSWR)

You can use an Antenna Analyser unit to measure impedance etc of an antenna. In most cases measuring and adjusting VSWR to a minimum is all that is needed



cheers
Dave

 

[Jason Holland]

Thanks for the quick response!

Midrange for the UHF band that I'm working on is 550 MHz. I've looked at yagi calculators, and I could build a relatively small one, but I am questioning just how directional it would be. Only one or two parasitic elements aren't likely to do much. Is there a reasonable way to shield the RF from the radiating element in such a way that it has a narrow beam?

 

[davenn]

and I could build a relatively small one,

what is your definition of relatively small ?

OK ....
1) is this for inside or outside use ?
2) why specifically do you want a highly directive receive antenna, as this goes against good operation of a moving around radio mic

Is there a reasonable way to shield the RF from the radiating element in such a way that it has a narrow beam?

that doesn't really make any sense ..... nothing you do with the coax will make the beamwidth wider or narrower


Dave

 

[Jason Holland]

I am an RF coordinator. I don't know much about antenna design/wiring, but I am heavily involved in frequency coordination for large events. I am trying to improve my knowledge base by digging into the engineering side of RF Tx and Rx.

The reason that I want a highly directional antenna is to identify and isolate unwanted sources of RF when working large venues. I was hoping to create something handheld mostly for indoor use.

 

[davenn]

The reason that I want a highly directional antenna is to identify and isolate unwanted sources of RF when working large venues.

That's going to be almost impossible ... ensuring you are using a frequency not used by anything else is a better bet
and as I said, it goes against what I said about picking up the microphone well as it moves around

I was hoping to create something handheld mostly for indoor use.

ok
so why don't you look at the many good quality commercial systems out there. They have their own RF freq allocation, 800 - 900 MHz,
they also have circuitry that helps with avoiding interference ... ie. most are digital spread spectrum

400 - 600 MHz is so full of other RF crap digital TV broadcasts, amateur radio, GPS base transmitter systems and a zillion other things

the 800-900 MHz radio mic allocation sits between the top end of the digital TV broadcast band and the bottom end of the cellular band


Dave

 

[davenn]

if you are working on this for large venues, you shouldn't be skimping on what is purchased and installed
and that's from personal experience

 

[Jason Holland]

I think I may be unclear. Here is what I'd like to do:

I would like to use my spectrum analyzer to isolate/identify unwanted RF interference. I would be using this antenna/spectrum analyzer setup mid show, when many wireless units are Txing. I am not trying to use this as an actual receiving element, as the wireless units have their own whips/log periodic antennas, front end and IF filters, and demodulators. I am using this as a troubleshooting tool. I have a few omnidirectional antennas, which I use for site surveys and collecting scan data. I use this information to coordinate my freqs, so by the time a show starts, I know what the spectrum should look like. I want something that can listen in the direction that I point it in, to find things like unauthorized transmitters, or to locate gear that is creating unintentional RF.

 

[davenn]

I think I may be unclear. Here is what I'd like to do:

I would like to use my spectrum analyzer to isolate/identify unwanted RF interference. I would be using this antenna/spectrum analyzer setup mid show, when many wireless units are Txing. I am not trying to use this as an actual receiving element, as the wireless units have their own whips/log periodic antennas, front end and IF filters, and demodulators. I am using this as a troubleshooting tool.

yup real unclear .... none of that was mentioned earlier

I want something that can listen in the direction that I point it in,.

yeah ok .... so a 4 - 5 element Yagi wouldn't be too unwieldy

. I want something that can listen in the direction that I point it in, to find things like unauthorized transmitters, or to locate gear that is creating unintentional RF.

and what do you do when you find something ? ... you will unlikely to know if the TX is unauthorised or not and if it is, you wont be able to do anything about it
as I said, that range of freqs is full of all sorts of stuff .... hence why the radio mic systems now have their own allocation

Dave

 

[Jason Holland]

The industry standard wireless microphone systems operate in the digital TV band. 470-698 MHz. There is a smaller band between 944-952 MHz that is occasionally seen on site, but most of our gear is packed in right on top of DTV. As you know, the white space (which is available for wireless microphone users) is dependent upon location, along with other sources of RF that are not in my control. This is why I scan before I coordinate.

I am often hired to coordinate frequencies for large events, where it is my responsibility to choose frequencies for all programmable wireless units that operate in this spectrum. I am the one who determines if a particular frequency is 'authorized'. An example would be a vendor attending a trade show who decides to bring his own wireless microphone, and turns it on assuming that it will just work. We end up taking 'hits' on mics that are properly coordinated due to the intermodulation products introduced by an uncoordinated transmitter. This has happened in the past on shows, and finding the offending transmitter is like finding a needle in a haystack.

Once a transmitter is found, it is my job to:
a)turn it off
or
b)fold it into my coordination so it will 'play nice' with the transmitters and receivers on site

Also, transmitters/receivers work just fine until certain pieces of gear (e.g. LED video walls) are introduced. Faulty equipment can also create unwanted RF, and can be difficult to find with an omni antenna. I know it's there, I just don't know where.

 

[davenn]

The industry standard wireless microphone systems operate in the digital TV band. 470-698 MHz. There is a smaller band between 944-952 MHz that is occasionally seen on site, but most of our gear is packed in right on top of DTV. As you know, the white space (which is available for wireless microphone users) is dependent upon location, along with other sources of RF that are not in my control. This is why I scan before I coordinate.

OK, obviously the USA has different allocations to many other countries
you have to work with what you have at hand

I am the one who determines if a particular frequency is 'authorized'.

and you have access to EVERY licenced transmitter from the FCC to determine that ?

An example would be a vendor attending a trade show who decides to bring his own wireless microphone, and turns it on assuming that it will just work. We end up taking 'hits' on mics that are properly coordinated due to the intermodulation products introduced by an uncoordinated transmitter.

This is my point, if he is using a radio mic in the correct band allocation, you cannot force him to turn off. All you can do is pick different channels on your system

Once a transmitter is found, it is my job to:
a)turn it off
or
b)fold it into my coordination so it will 'play nice' with the transmitters and receivers on site

yeah, that is really your only choice, if the other transmitters are legal

Also, transmitters/receivers work just fine until certain pieces of gear (e.g. LED video walls) are introduced. Faulty equipment can also create unwanted RF, and can be difficult to find with an omni antenna. I know it's there, I just don't know where.

uh huh, been there done that can be a lot of fun ... not


D

 

[Jason Holland]

I appreciate your perspective Dave, but most of this conversation is irrelevant. When my client rents a space, and hires me to coordinate all of the transmitters in that space, we decide who can operate gear in that space. The FCC may say 'yeah, go ahead and use that freq', but the client hosting the event can say 'don't use that'. The client often knows little about coordination, which is why they hire me. Having a license to transmit from the FCC does not mean that you can go wherever you want and transmit. If I show up at your house with a transmitter, my FCC license is worthless if you simply ask me to turn it off, change it or leave.

All of that being said, I am still trying to build a high gain antenna that operates in the UHF band, tuned to approx 550 MHz.

 

[duke37]

Go for a Yagi, the longer and the more elements, the bigger the gain.
The antenna has no power gain, the gain specified is due to the restricted area sensitivity.A good design will have a large, narrow main lobe and very little sensitivity in minor lobes. The width of the lobe is related to the gain, narrow lobe, high gain.

The antenna should be matched to the feeder to eliminate reflections in the feeder and degradation of the performance. Your SWR meter will tell if you have a good match but it may be difficult to adjust for best match without more instrumentation.

 

[Jason Holland]

Thanks for the info Duke. I was hoping that there was some better option than a yagi. I am more interested in signal rejection than I am in "gain". I am aware that a directional antenna is called a "high-gain" antenna, not because it boosts power, but because of its directivity. I was hoping that there was a compact design that is better or equal to a yagi... perhaps there's a way to design a waveguide for this band without taking up too much space?

 

[davenn]

perhaps there's a way to design a waveguide for this band without taking up too much space?

a WG for 500MHz would be huge and heavy, stick with the 3 element yagi, it will give you a reasonable directional function

 

[Jason Holland]

If anyone is interested, I ended up going with a log periodic. This gives me the best gain to bandwidth to size ratio. Thanks for all of your input and advice!

Cheers,
-Jason