Antenna beamwidth

[paulears]

Also worth looking at Diamond capacitors - they had a few batches that were adversely affected by higher power outputs, and while they look OK, a prod sees them crumble. I've had 3 of the shorter ones do this now.

The 7.8dB claim is 7.2dBi I'm pretty certain - from some older specs I still have - at UHF. The same antenna, operating as 3 x ¼ waves at VHF is a bit further down at 4.5dBi - I actually thought the VHF figure looked a bit low, with 4.5dBd being more likely, for the design, but that would make the 7.2 figure higher? which equally seems wrong!

 

[Ubbe]

Is there a way to test the antenna?

As you deal a lot with antennas and probably need to check on them from time to time, then a VNA could be handy. To properly use one you'll almost need to be a RF engineer, but if just getting a calibration kit and run that process and connect an antenna you can see what frequency it's tuned to and if the SWR are good. And you can also see if the coax are fine and the connectors on the way if they have low loss and if the coax have been squashed or have a crease somewhere. You can also check and tune bandpass and notch filters.

I can't advice which one to get but all the cheap ones are very sensitive to any signals in the air that the antenna receives as it upsets the measurements. So one has to be careful with those cheap ones as you can easily get false readings when the antenna are "live" and receive high level signals.

/Ubbe

 

[paulears]

I have a Rigol analyser and am often surprised what you see with a tracking generator - I did a test years back now before I got the analyser with loads of hand-held antennas and was surprised how awful some were. I've never stuck these on an analyser, but I suspect that the result would be loads of humps and spikes all over the place where unintended and quite random peaks and troughs in the performance are. The simple quarter waves tend to be just as you'd expect, a gradual increase in efficiency when you approach the cut length, then the same roll off. The mess that dual banners, and a quad band I have on my mast generate makes you work hard to spot where the resonance points are! It's very common to see antennas that should be for say 430-440 and 146-148 also having useful reception in the military air band, by sheer luck. However, some of them have sizeable notches at frequencies in the marine band, or the UHF (in the UK) business radio portion of the band. 6dB or more down in a 'useful' area but you don't know until you stick it into the analyser. My old discone I had a friendly feeling about, even though it's approaching 40 years old proved to be really good - lots of gentle peaks and troughs, but none remotely 'dead' and none exceptionally high - so while it has no gain, it has a little less than a dipole ...... everywhere - ideal for scanning. Until I bought and analyser the best you could do would be to measure VSWR at loads of frequencies and try to kind of join the dots to see the curves change. Those little Chinese analysers seem to do a reasonable job - I've got one somewhere but at the moment it is lost!

 

[W8HDU]

A lot of times in multiple frequency antennas you give up a good impedance for the convenience and performance. Typically, the wider the spread of frequencies, and gain, the more the problem manifests on the impedance side.
For transmit it's a problem, but receiving not so much.
Imagine what we go though in TV broadcast trying to keep a decent match over 6 MHz, or two adjacent channels (12 MHz) yet maintain a match.

 

[Ubbe]

A lot of times in multiple frequency antennas you give up a good impedance for the convenience and performance.

I modified my Diamond X510 and removed its SWR network as it attenuated too much. I now have 6:1 SWR in VHF and 4:1 in UHF but receives signals on all frequencies much better, even at 70MHz, and still works just as good at 145MHz and 433MHz as it did with the SWR net in place, as the vertical elements are still tuned to those frequencies.

/Ubbe

 

[Rawkee1]

I conversed with some hams. some say go with a high gain antennas when some told me go with unity gain. I’m sure there both right in their own way, but what’s better for receiving signals only for Scanner listing. one guy said unity gain gives less distortion with clean sound.

 

[MDScanFan]

The statement " one guy said unity gain gives less distortion with clean sound " is incorrect. For rail signals a higher gain towards the signal of interest will increase the signal level and improve weak signal reception.

The posts early in this thread provide some insight into the considerations for high gain antennas. Unless your antenna is very high gain, or you have a weird geometry situation, then you should only see improvement in weak signal reception using the higher gain omni antennas discussed in this thread, such as the Laird antenna or a four bay antenna.

I conversed with some hams. some say go with a high gain antennas when some told me go with unity gain. I’m sure there both right in their own way, but what’s better for receiving signals only for Scanner listing. one guy said unity gain gives less distortion with clean sound.

 

[sonm10]

I conversed with some hams. some say go with a high gain antennas when some told me go with unity gain. I’m sure there both right in their own way, but what’s better for receiving signals only for Scanner listing. one guy said unity gain gives less distortion with clean sound.

The statement " one guy said unity gain gives less distortion with clean sound " is incorrect. For rail signals a higher gain towards the signal of interest will increase the signal level and improve weak signal reception.

The posts early in this thread provide some insight into the considerations for high gain antennas. Unless your antenna is very high gain, or you have a weird geometry situation, then you should only see improvement in weak signal reception using the higher gain omni antennas discussed in this thread, such as the Laird antenna or a four bay antenna.

It's not just the gain, but the pattern. A higher gain antenna will "squash" the signal to a lower take off angle. This will pull more signal from the horizon. Good for flat land.

A unity gain antenna, the pattern is straight up. These are better suited for mountainous regions, trying to pull signal from over a mountain.

 

[Ubbe]

If a mountain are blocking the signal it will usually get reflected from objects on its ridge, like trees and rocks and houses, and the antenna needs to be able to receive that signal up on the ridge. Sometimes my neighbours house behind me gets a cleaner signal and when turning my directional yagi antenna the opposite way at my neighbour I get a stronger signal from the reflection from his house. When using an omni antenna the direct signal and that reflected signal from my neighbours house can be in opposite phases and could cancel each other out.

Each location will have its unique problems and solutions and one general fix do not apply to all.

/Ubbe

 

[Rawkee1]

Ok guys, I got the picture. That would answer why the Hot Box Detector comes in faintly some days and stron other days. Also that problem with NOAA coming over my RS 2067 when the 996P2 receives on 472.3125 and 154.385. Does’t happen all the time. I’m thinking when the sun hits a building a block away, it reflects and hits my antenna area big time. Could that be the culprit I wonder that’s causing some stuff?

 

[prcguy]

The sun or a reflection from the sun has nothing to do with your reception. If your not line of sight to what your receiving and relying on reflections, the reflecting surfaces will change during the day, night and seasons. Moisture will effect reflections off hills and foliage. A change of seasons will have more or less or no leaves on trees which absorb higher frequencies.

Ok guys, I got the picture. That would answer why the Hot Box Detector comes in faintly some days and stron other days. Also that problem with NOAA coming over my RS 2067 when the 996P2 receives on 472.3125 and 154.385. Does’t happen all the time. I’m thinking when the sun hits a building a block away, it reflects and hits my antenna area big time. Could that be the culprit I wonder that’s causing some stuff?

 

[Rawkee1]

How would you compare the Laird Fg1563 3db to the Telewave ANT150D VHF base antenna? Even used, the Telewave is a bit pricey but if one out preformed the other, I’d be all in. I see a lot of the Telewave single, double stack, and even quad stacked when I’m driving around. One train yard has a quad Telewave and the csx yard has a few Laird 1/2 wave antennas and some yagis to boot. The Telewave 150D looks like it should be easy to build unless I’m missing something.

 

[prcguy]

The Laird is rated 3dBd gain and the Telewave is a 1/2 wave dipole with roughly 0dBd gain in free space and no mast. As soon as you put it on a mast it will become directional and depending on the spacing from the mast it can be a cardioid pattern or something close to a figure 8. It will have a little gain in the direction of the lobes.

If you only need 180 deg or less coverage then the Telewave with the right spacing might be on par with the Laird.

How would you compare the Laird Fg1563 3db to the Telewave ANT150D VHF base antenna? Even used, the Telewave is a bit pricey but if one out preformed the other, I’d be all in. I see a lot of the Telewave single, double stack, and even quad stacked when I’m driving around. One train yard has a quad Telewave and the csx yard has a few Laird 1/2 wave antennas and some yagis to boot. The Telewave 150D looks like it should be easy to build unless I’m missing something.