Measuring Antenna Gain
Posted by: Mike Hanson on 16 April 2004
Up until yesterday, I've been using a regular free-in-the-box antenna (I believe it's just a strip of 300-ohm cable), and it's basically worked fine. Being only a few miles from the CN Tower in Toronto, the signals tend to be very strong, so not much amplification is needed.
However, occasionally I would get static on my favorite stations, and some stations would have a slight "edge" to them. Last night I went to Radio Shack to buy some of their rabbit ears, and the results were somewhat mixed.
Most stations sound a bit clearer and fuller. Some of the stations that had the edge don't now, but others have acquired a new one. I'm not certain whether I'm getting too much gain now, or not enough, or whether directionality is having an impact. I believe that both antennae are aimed in the same general direction.
It's rather hard to find stations on my dial that are "marginal", which might enable me to compare the two antennae. As an alternative, is there an easy way to test the gain of an antenna? Can I use a regular digital multimeter to check the voltage, or would the reading be too small? Are the appropriate meters generally available for rent? If not, who do I look up in the phonebook for help (if I decide to go that far)?
BTW, many in Toronto don't even use an area. I believe Vuk, who lives about half as far from the CN Tower, is one example. He actually has a Magnum Dynalab whip that he brought from London, Ontario, but he found that it didn't make things any better, so now he runs without. Therefore, I really don't need much gain.
Coincidentally, I've got a Ron Smith Galaxy 17 in a box downstairs, but that would certainly be overkill. Dev once suggested that I setup just the final collector (without all of the amplifying elements), but I haven't investigated that yet. Perhaps I'll dig that out and see if that makes things better or worse.
BTW, I'm using a Creek T32, which shows "3-bars" (i.e. full strength) for all of the stations that I've tested, regardless of which antenna is used. If the signal is too strong, is the tuner likely to attenuate it automatically?
Sorry for all of the questions, but I'm a bit of a newbie in this area.
-=> Mike Hanson <=-
However, occasionally I would get static on my favorite stations, and some stations would have a slight "edge" to them. Last night I went to Radio Shack to buy some of their rabbit ears, and the results were somewhat mixed.
Most stations sound a bit clearer and fuller. Some of the stations that had the edge don't now, but others have acquired a new one. I'm not certain whether I'm getting too much gain now, or not enough, or whether directionality is having an impact. I believe that both antennae are aimed in the same general direction.
It's rather hard to find stations on my dial that are "marginal", which might enable me to compare the two antennae. As an alternative, is there an easy way to test the gain of an antenna? Can I use a regular digital multimeter to check the voltage, or would the reading be too small? Are the appropriate meters generally available for rent? If not, who do I look up in the phonebook for help (if I decide to go that far)?
BTW, many in Toronto don't even use an area. I believe Vuk, who lives about half as far from the CN Tower, is one example. He actually has a Magnum Dynalab whip that he brought from London, Ontario, but he found that it didn't make things any better, so now he runs without. Therefore, I really don't need much gain.
Coincidentally, I've got a Ron Smith Galaxy 17 in a box downstairs, but that would certainly be overkill. Dev once suggested that I setup just the final collector (without all of the amplifying elements), but I haven't investigated that yet. Perhaps I'll dig that out and see if that makes things better or worse.
BTW, I'm using a Creek T32, which shows "3-bars" (i.e. full strength) for all of the stations that I've tested, regardless of which antenna is used. If the signal is too strong, is the tuner likely to attenuate it automatically?
Sorry for all of the questions, but I'm a bit of a newbie in this area.
-=> Mike Hanson <=-
quote:
Originally posted by bazz:
The G17 if I recall has about 16db gain, which is a lot. It is very possible that the signal may overload your tuner. That's easy to pick by the spitty over-modulated sound.
This perfectly describes how one of my favourite stations changed with the new antenna, so I may already have a signal that's too strong.
Most of the stations are very crystal clear. In fact, my humble Creek T43 often seems to rival the "magic" of my CDS2 (with either antenna). Don't ask me to explain that one.
Installing the G17 is a major undertaking, and I don't know if it's worth my trouble (especially if I already have sufficient signal strength). I'm almost line-of-sight to the CN tower, which is the source of the vast majority radio stations here in Toronto. The only others are in Hamilton (directly beyond the CN Tower, line of sight) and Buffalo (90 degrees south, but about 50 miles across the lake).
I'm trying to find a good antenna expert in the area, but I'm not entirely sure how to find one. What does one lookup in the yellow pages? Or can anyone suggest one in the Toronto area?
-=> Mike Hanson <=-
quote:
Originally posted by James:
If you have no need for the G17, would you consider selling it to me? If so, please PT me and we'll discuss offline.
I originally got it for a pittance on ebay, and it was purchased as a "just in case" option. At the time, I was still living in an apartment, and I wasn't sure whether I would be purchasing a house in the city proper, out of town. Now that I'm in-town, I'll have to reconsider whether I'm going to need it for a while. Even if I moved out to the country in 10 years, radio may not be analog anymore.
I'll know soon whether I'm willing to sell it, and I'll let you know.-=> Mike Hanson <=-
Antenna's, my specialist subject 
Mike,
In answer to you initial question there isn't any simple way to measure antenna gain. Whilst looking at received signal strength can give an indication, it's perfectly feasible, given suitable multipath conditions, for a lower gain antenna to apparently receive a stronger signal.
To achieve antenna gain you have to make a trade omnidirectional characteristics for a more 'focussed' beam. This is why most antenna have gain figures measured relative to either a dipole antenna (dBd) or a fictitious isotropic point source (dBi).
There are two ways to do this (which can be combined); you can reduce the horizontal beamwidth in the way most multi-element antenna do (Yagi types, usually) or the vertical beamwidth can be reduced, usually via several phased dipoles know as a collinear array.
For FM we want to restrict the horizontal beamwidth, to provide gain to the intended transmission, but attenuate any multipath signals (i.e. signals that are reflected off of surrounding buildings / structures etc.). For good FM reception minimising multipath is a key goal.
To this end we normally use the Yagi style of antenna (of which the Galaxie antennae are a variant). These have one driven element, in the form of a dipole, or folded dipole, a reflector (to reduce the signals from behind the antenna) and numerous passive elements in front to focus. The Galaxie range also appear to be, in effect, a stacked array, vertically, so they also benefit from additional gain by compressing the vertical beamwidth.
Ron Smith offers excellent and technically sound advice on optimising FM reception and I wholly endorse his approach. The key elements are: -
1. An antenna with excess gain, providing a signal in many cases that is too strong for the tuner - this can readily be attenuated and there are good reasons for doing so, which I'll mention later.
2. Sufficent directionality in the antenna to attenuate multipath (unwanted) signals, with suitable antenna positioning. This sometimes means that the antenna is not pointed directly at the transmitter, but at an angle that minimises any unwanted reflections, whilst keeping the transmitter in the beamwidth of the antenna.
2. Double-screened, high quality, low loss (satellite grade) coaxial cable - in long cable runs the coax is a significant source of signal pickup, which we want to avoid, double screening helps, along with signal attenuation at the tuner.
The benefit to starting with a strong signal, then attenuating it is this; the cable is a fixed source of signal pickup, which we want to minimise. Without attenuation we get the antenna signal + the cable signal at the tuner.
If we start with excess signal and then attenuate at the tuner we get a bigger antenna signal + the same cable signal, but when attenuated we end up with the same antenna signal at the tuner, yet a smaller cable signal. Clever, huh?
There's a secondary benefit too, the attenuator provides some isolation between the cable / antenna and the tuner, this means the tuner 'sees' a more ideal impedance load as will the antenna / cable, this further optimises performance of both the tuner and the antenna, since both are designed assuming a perfect 75ohm match, which in practice one never achieves.
Finally I would not endorse removing the additional elements from the Galaxie, it will affect the match of the antenna and whilst it may be possible to get good results, the antenna is unlikely to function remotely as intended.
Andy.
[This message was edited by Andrew L. Weekes on Mon 19 April 2004 at 10:45.]
Mike,
In answer to you initial question there isn't any simple way to measure antenna gain. Whilst looking at received signal strength can give an indication, it's perfectly feasible, given suitable multipath conditions, for a lower gain antenna to apparently receive a stronger signal.
To achieve antenna gain you have to make a trade omnidirectional characteristics for a more 'focussed' beam. This is why most antenna have gain figures measured relative to either a dipole antenna (dBd) or a fictitious isotropic point source (dBi).
There are two ways to do this (which can be combined); you can reduce the horizontal beamwidth in the way most multi-element antenna do (Yagi types, usually) or the vertical beamwidth can be reduced, usually via several phased dipoles know as a collinear array.
For FM we want to restrict the horizontal beamwidth, to provide gain to the intended transmission, but attenuate any multipath signals (i.e. signals that are reflected off of surrounding buildings / structures etc.). For good FM reception minimising multipath is a key goal.
To this end we normally use the Yagi style of antenna (of which the Galaxie antennae are a variant). These have one driven element, in the form of a dipole, or folded dipole, a reflector (to reduce the signals from behind the antenna) and numerous passive elements in front to focus. The Galaxie range also appear to be, in effect, a stacked array, vertically, so they also benefit from additional gain by compressing the vertical beamwidth.
Ron Smith offers excellent and technically sound advice on optimising FM reception and I wholly endorse his approach. The key elements are: -
1. An antenna with excess gain, providing a signal in many cases that is too strong for the tuner - this can readily be attenuated and there are good reasons for doing so, which I'll mention later.
2. Sufficent directionality in the antenna to attenuate multipath (unwanted) signals, with suitable antenna positioning. This sometimes means that the antenna is not pointed directly at the transmitter, but at an angle that minimises any unwanted reflections, whilst keeping the transmitter in the beamwidth of the antenna.
2. Double-screened, high quality, low loss (satellite grade) coaxial cable - in long cable runs the coax is a significant source of signal pickup, which we want to avoid, double screening helps, along with signal attenuation at the tuner.
The benefit to starting with a strong signal, then attenuating it is this; the cable is a fixed source of signal pickup, which we want to minimise. Without attenuation we get the antenna signal + the cable signal at the tuner.
If we start with excess signal and then attenuate at the tuner we get a bigger antenna signal + the same cable signal, but when attenuated we end up with the same antenna signal at the tuner, yet a smaller cable signal. Clever, huh?
There's a secondary benefit too, the attenuator provides some isolation between the cable / antenna and the tuner, this means the tuner 'sees' a more ideal impedance load as will the antenna / cable, this further optimises performance of both the tuner and the antenna, since both are designed assuming a perfect 75ohm match, which in practice one never achieves.
Finally I would not endorse removing the additional elements from the Galaxie, it will affect the match of the antenna and whilst it may be possible to get good results, the antenna is unlikely to function remotely as intended.
Andy.
[This message was edited by Andrew L. Weekes on Mon 19 April 2004 at 10:45.]
Thanks for the in-depth response, Andy! It was very helpful. I think I've read some of that stuff before, and this refresher has helped me to clarify my thoughts.
Since it's a HUGE visual structure, I would have to get my wife's acceptance before proceeding with the installation. (I would also take some convincing myself, as I think the antenna is even less attractive than the oil-rig-inspired Mana.)
She would also have to be convinced that it "solves a problem", and with the radio currently sounding pretty good, that will probably be a tough row to hoe. There's also the fact that I rarely sit and listen intently while the radio is playing. Instead, it's usually a background music source, while I'm moving around the house and don't feel like changing CDs. Consequently, I'm not that I "need" it to sound optimal.
I would also have to find someone locally who can do these types of installations, and see how much they would charge me for it. Since Ron Smith's products are not common here in Canada, that might challenging.
Although I can appreciate the benefits of the big antenna, there's a good chance that it won't happen.
-=> Mike Hanson <=-
[This message was edited by Mike Hanson on Mon 19 April 2004 at 12:13.]
Since it's a HUGE visual structure, I would have to get my wife's acceptance before proceeding with the installation. (I would also take some convincing myself, as I think the antenna is even less attractive than the oil-rig-inspired Mana.)
She would also have to be convinced that it "solves a problem", and with the radio currently sounding pretty good, that will probably be a tough row to hoe. There's also the fact that I rarely sit and listen intently while the radio is playing. Instead, it's usually a background music source, while I'm moving around the house and don't feel like changing CDs. Consequently, I'm not that I "need" it to sound optimal.
I would also have to find someone locally who can do these types of installations, and see how much they would charge me for it. Since Ron Smith's products are not common here in Canada, that might challenging.
Although I can appreciate the benefits of the big antenna, there's a good chance that it won't happen.
-=> Mike Hanson <=-
[This message was edited by Mike Hanson on Mon 19 April 2004 at 12:13.]