Increase Gain of Beam Antenna with Plastic Pipe

[Neutrino222]

I have a beam antenna tuned to 400-470mhz. It has a gain of 8db. I had the idea in my mind that this could be increased by cutting a pipe in half - longways - and lining the inside with tinfoil. This would then be placed somewhere behind the reflector, which I guess would need some experimentation to find the optimal distance.

Is my thinking technically feasible for such a setup? Has anyone else modified their own antenna with a view to increasing the gain?

There's nothing particular wrong with my setup; I have 30 feet of pre-made RG 58, and both the centre copper and shielding passed my continuity test. I'd just like to grab a little more distance, so I can hear the next few towns and cities.

 

[KB1VLA]

I would start with upgrading your coax. 30' of RG58 has a 3db loss at 460 MHz, meaning you are losing half of the incoming signals in the coax. Using RG213 or LMR400 would be a much better choice for UHF frequencies.

 

[buddrousa]

You will gain more signal with better COAX 9913 than play with the beam plastic and tinfoil.

 

[prcguy]

No, that won't do anything. If it did the mfr would have done it already. There was a "new" Yagi reflector design in the 1970s from the National Bureau of Standards that had two additional reflectors behind and to the sides of the first reflector. This improved front to back and maybe added a tiny amount of gain. Page 3 of this paper shows the reflector design and some companies adopted it but I haven't seen any recently so apparently its not worth all the effort.

https://tf.nist.gov/general/pdf/451.pdf

I have a beam antenna tuned to 400-470mhz. It has a gain of 8db. I had the idea in my mind that this could be increased by cutting a pipe in half - longways - and lining the inside with tinfoil. This would then be placed somewhere behind the reflector, which I guess would need some experimentation to find the optimal distance.

Is my thinking technically feasible for such a setup? Has anyone else modified their own antenna with a view to increasing the gain?

There's nothing particular wrong with my setup; I have 30 feet of pre-made RG 58, and both the centre copper and shielding passed my continuity test. I'd just like to grab a little more distance, so I can hear the next few towns and cities.

 

[mmckenna]

As others pointed out, you can increase your signals by almost 2dB really quickly by swapping out that RG-58 to something like LMR-400:

30 feet @ 470MHz:
RG-58 = 2.755dB loss (depending on the exact type of cable)
LMR-400 = 0.825dB loss

 

[Ubbe]

I have a beam antenna tuned to 400-470mhz. It has a gain of 8db.

What's the possibility of raising you antenna further up above ground? If you already are above roof tops and the top of trees it wont help much but if the antenna are lower than that it will help a lot.

If you add an amplifier at the antenna you will overcome any coax and connector losses, even that 0,8dB of LMR400 losses. Your scanner has probably an internal noise figure of at least 3-4dB and if you use a 1dB NF amplifier you will add an additional 6dB of gain to your current setup. That's equal to making your antenna 4 times longer. That amplifier add-on are probably a $50 cost and compare that to the purchase and installation time of LMR400 coax. If you go for the amplifier you will have to attenuate the signal at around 10dB at the scanner. Best are to use a variable 0-20dB attenuator to get the exact setting where you have the best reception of weak signals.

/Ubbe

 

[buddrousa]

LMR400 can be bought for 1 Dollar per foot and is a known improvement the OP has 30 foot that is a know 20 dollar savings.

 

[Neutrino222]

As others pointed out, you can increase your signals by almost 2dB really quickly by swapping out that RG-58 to something like LMR-400:

30 feet @ 470MHz:
RG-58 = 2.755dB loss (depending on the exact type of cable)
LMR-400 = 0.825dB loss

LMR 400 is quite expensive. I found 3 meters cost £48. Looking at the figures, I guess I see why.

 

[Neutrino222]

LMR400 can be bought for 1 Dollar per foot and is a known improvement the OP has 30 foot that is a know 20 dollar savings.

I can only seem to find very expensive LMR 400's here in UK. £48 for 3 meters. Mind you, they come pre-installed with the connections.

 

[Neutrino222]

What's the possibility of raising you antenna further up above ground? If you already are above roof tops and the top of trees it wont help much but if the antenna are lower than that it will help a lot.

If you add an amplifier at the antenna you will overcome any coax and connector losses, even that 0,8dB of LMR400 losses. Your scanner has probably an internal noise figure of at least 3-4dB and if you use a 1dB NF amplifier you will add an additional 6dB of gain to your current setup. That's equal to making your antenna 4 times longer. That amplifier add-on are probably a $50 cost and compare that to the purchase and installation time of LMR400 coax. If you go for the amplifier you will have to attenuate the signal at around 10dB at the scanner. Best are to use a variable 0-20dB attenuator to get the exact setting where you have the best reception of weak signals.

/Ubbe

Interesting! I'll look into the amplifier. Thanks.

 

[Ubbe]

I can only seem to find very expensive LMR 400's here in UK. £48 for 3 meters. Mind you, they come pre-installed with the connections.

LMR400 should be something like £3 per meter, but connectors can be expensive and fitting your own usually requires special and expensive tools. So best to have them fitted by the seller. But with an amplifier you can still use your RG58.

Do a search on Ebay for PGA103+ as those amplifiers have the best performance for a low cost. Then check how you are gonna power it, by a separate wire or thru the coax, that then need a bias-T module. The variable attenuator are a £7 item in UK. And you can just as well get a £5 CATV splitter to have more ports for other receivers. There's an Hungarian guy that have different stuff for sale that differs from time to time, and also from Greece and other places. Items for sale by janilab | eBay

/Ubbe

 

[Neutrino222]

LMR400 should be something like £3 per meter, but connectors can be expensive and fitting your own usually requires special and expensive tools. So best to have them fitted by the seller. But with an amplifier you can still use your RG58.

Do a search on Ebay for PGA103+ as those amplifiers have the best performance for a low cost. Then check how you are gonna power it, by a separate wire or thru the coax, that then need a bias-T module. The variable attenuator are a £7 item in UK. And you can just as well get a £5 CATV splitter to have more ports for other receivers. There's an Hungarian guy that have different stuff for sale that differs from time to time, and also from Greece and other places. Items for sale by janilab | eBay

/Ubbe

Thank you. I bought 32 feet of Paradar X-400, which is basically LMR-400. Seems to have great reviews. With this cable, I could probably hear a little more, but would it be worth adding an amplifier with such a small signal loss?

 

[Ubbe]

A scanner have something like an internal noise figure of 5-6dB. If you have a sensitivity of -115dBm in that scanner then you will not hear a weaker -120dBm signal. If you have an amplifier with a 1dB noise level and that receives the -120dBm signal and amplifies it 5dB to -115dBm then your scanner will receive that -120dBm antenna signal. Add the coax and connector loss, add the impedance difference of the antenna at different frequencies that adds loss and that the amplifier gives a constant 50 ohm to the coax, then you gain a little bit more in signal level that you can receive.

A scanners bandpass filters are not a constant 50 ohm over its range, it can be 25-200 ohm. Add a splitter that isolates the scanner and gives a constant impedance to the coax and you get a little less signal loss, even if you use a 75 ohm CATV splitter. Add a variable attenuator and you can probably add a little extra gain to boost that 120dBm above -115dBm up to -112dBm to get a more noise free signal.

There are situations where amplifiers doesn't work. If you have a strong transmitter in the same band that might be received at a -40dBm level, then adding gain will probably overload the scanners amplifier and it starts to loose sensitivity. Add even more gain and it starts to create intermodulation products. That's why a variable attenuator are useful to dial in the perfect gain.

So even having that amplifier directly at the scanner will improve reception due to its lower noise figure. But the antenna will still load the coax with different impedances at different frequencies creating SWR in the coax down to the amplifier/scanner.

That X400 coax has less than 1,3dB loss at 30ft @433MHz. Your RG58 probably have up to 3,5dB loss, it's a 2dB difference.

/Ubbe

 

[Neutrino222]

A scanner have something like an internal noise figure of 5-6dB. If you have a sensitivity of -115dBm in that scanner then you will not hear a weaker -120dBm signal. If you have an amplifier with a 1dB noise level and that receives the -120dBm signal and amplifies it 5dB to -115dBm then your scanner will receive that -120dBm antenna signal. Add the coax and connector loss, add the impedance difference of the antenna at different frequencies that adds loss and that the amplifier gives a constant 50 ohm to the coax, then you gain a little bit more in signal level that you can receive.

A scanners bandpass filters are not a constant 50 ohm over its range, it can be 25-200 ohm. Add a splitter that isolates the scanner and gives a constant impedance to the coax and you get a little less signal loss, even if you use a 75 ohm CATV splitter. Add a variable attenuator and you can probably add a little extra gain to boost that 120dBm above -115dBm up to -112dBm to get a more noise free signal.

There are situations where amplifiers doesn't work. If you have a strong transmitter in the same band that might be received at a -40dBm level, then adding gain will probably overload the scanners amplifier and it starts to loose sensitivity. Add even more gain and it starts to create intermodulation products. That's why a variable attenuator are useful to dial in the perfect gain.

So even having that amplifier directly at the scanner will improve reception due to its lower noise figure. But the antenna will still load the coax with different impedances at different frequencies creating SWR in the coax down to the amplifier/scanner.

That X400 coax has less than 1,3dB loss at 30ft @433MHz. Your RG58 probably have up to 3,5dB loss, it's a 2dB difference.

/Ubbe

Thank you, Ubbe. That's really helpful info. By the way, is there an incremental relationship between the RSSI and dB? In other words, how does 1dB translate into an RSSI figure?

 

[Ubbe]

In other words, how does 1dB translate into an RSSI figure?

dB are how much difference there is. When a dB values zero are in reference to something it gets an additional letter. 0 dBm are referenced to 1 milli Watt.

Each 10dB are a 10 times change. So -10dBm are 0,1 mW and -20dBm are 100 times lower from 0dBm at 0,01mW and -30dBm are a thousand times lower at 0,001mW.

If an antenna receives a signal at -100dBm and it gets replaced by an antenna that has 3dB more gain it should be receiving the signal at -97dBm.
dBi are referenced to a theoretical isotropic antenna that radiate the same in all directions. dBd are a reference to a 1/2 wave dipole antenna.
If an antenna specification only says dB, then they could calculate with the ground effect that signals bounce on the ground and then up to the antenna and adds to the signal level, and that makes it difficult to compare to other antennas if the do not state dBi or dBd.

/Ubbe