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HYS 11 db Yagi receiving signals from all directions


WRZC903

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Yagis are directional, but they don't completely reject signals from the sides or back. Many times I've heard HF radio operators talking to people hundreds of miles away off the sides or back of their Yagi antennas. Using my 3-element handheld VHF Yagi, I can often hear the repeater I'm using even off the side or rear of the Yagi, just not as well as when I'm pointed at it. There's not much to go wrong with a Yagi, so I wouldn't think it's defective. In fact, the idea that it's actually receiving less off the sides makes me think it's working just fine.

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4 hours ago, WRZN263 said:

Can anyone point me to the FCC rules on using a Yagi on GMRS channels? 

There are no antenna restrictions other than heights and near airports, per Rule Section 95.1741. You can use all the ERP you want providing the radio transmitter does not exceed the RF output of 50 watts on the main channels for mobiles, base, and repeaters; and 15 watts for fixed stations; and 5 watts ERP on the 462 MHz interstitial channels; and 0.5 watt ERP on the 467 MHz interstitial channels.

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2 hours ago, BoxCar said:

@Sshannon Actually, you need to cite the entirety of Title 47 as the question was regarding type not placement or installation. There are no rules regarding when a Yagi antenna may be used while there are rules for the installation covering all antennas regardless of type.

Okay: https://www.ecfr.gov/current/title-47

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There is just SO much wrong with this.

Gonna go full on with the math here so it makes sense.  And this is gonna be based on that if the antenna in question has 11dB of forward gain that it has an 11dB null off some part of the pattern, most likely the back side off the corners.

Second assumption is the repeaters in question are running full legal limit power of 50 watts minus a standard combined loss of 3 dB for feedline and duplexer losses and a 6dB gain antenna.

So conversion of 50 watts is 47.0 dBm for ease of the math.  Real conversion of 50 watts is 46.9897dBm so I rounded up to save time.  Mind you that a 3dB change is a doubling or halfing of power.  So, 47 dBm minus 3dB is of course 44dBm OR 25.11 watts.  Getting that out of the way.

So we have a 44dBm signal going into a repeater antenna with 6dB of gain.  So that's 50dBm or 100 watt's ERP if you want to go that route.  ERP is Effective Radiated Power.  Look it up for a better explanation.

 

Now we need to consider path loss.  Path loss is the amount of signal drop between the two antenna's ( repeater transmitting antenna, and your receiving antenna).

The path loss calculator used for these calculations.

https://www.pasternack.com/t-calculator-fspl.aspx?utm_campaign=Power_Combiners&keyword=&gad_source=1&gclid=Cj0KCQiAkKqsBhC3ARIsAEEjuJj7ApOIs_PMeM7_yH6hraxqwaoC9i42tDkgTTaSKcF2Q5qN0GSO0coaAtBGEALw_wcB

Path loss on a 9 mile path for 462.550Mhz is 109dB.

So we take 50dBm and subtract 109dB which leaves up with -59dBm.

Your receiver will open with a tight squelch setting at about -110 to -105dBm signal level. 

So lets add the null to the -59dBm signal level and we get -70dBm.  This is when the signal null is the greatest when the antenna null is turned to face the transmitted signal.

So your radio will open up at -110 to -105dBm and you have a signal level of -70 dBm at the antenna.  Even with another 3dB of signal loss in your coax feeding the receiver, you STILL have a -73dBm signal level hitting your receiver.  To put into contrast the difference between -103 and -73 dBm which is 30dB of course.

If you have a 1 watt transmitter, and you amplify the 1 watt by 30dB, you now have 1000 watts.  It's THAT BIG of a change in signal level.  And your receiver is hearing that much more signal than it needs to in order to open the squelch up and hear the signal. 

So, is your yagi defective?  I am gonna say NO.  And this is the math that proves it's working. 

 

If you ever worndered how the FCC can sit in your neighborhood and gell how much power your CB radio is putting out without needing to walk in and test it with a watt meter, these equations are it.  You measure signal level off of a calibrated antenna into a signal level meter or spectrum analyzer and calculate it back.  Your antenna of course is clearly visible above your house, and it's design will indicate a reasonable gain number to apply to the math to figure out just how much fire you are putting in the wire. 

And mind you, don't take my word for all this.  There are two RF engineers on this board,  One has posted in this thread.  If he chooses to pipe up he can verify this math is correct and why you are hearing the signal off the side of your beam.  And also verify that even if the repeater output was reduced to 1 watt, you would STILL hear it off the side of your beam.  The math don't lie.    I would ask him, if he reads this, that if it's the case, that he bump the answer over on the top left of the post here.  It makes my numbers look a bit better.  And I will continue to do that for him as well. 

Gonna toss this link in here too, gives a better explanation of a yagi and how it effects signal pattern for both receive and transmit. 

 

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Not necessarily. It may mean that the transmitter source may be very nearby and saturating the antenna with a high signal level. If you put an attenuator in-line of a high value, maybe 40-60 dB, you should see a significant difference in the performance and the yagi performing more directional.

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7 hours ago, WRKC935 said:

There is just SO much wrong with this.

Gonna go full on with the math here so it makes sense.  And this is gonna be based on that if the antenna in question has 11dB of forward gain that it has an 11dB null off some part of the pattern, most likely the back side off the corners.

Second assumption is the repeaters in question are running full legal limit power of 50 watts minus a standard combined loss of 3 dB for feedline and duplexer losses and a 6dB gain antenna.

So conversion of 50 watts is 47.0 dBm for ease of the math.  Real conversion of 50 watts is 46.9897dBm so I rounded up to save time.  Mind you that a 3dB change is a doubling or halfing of power.  So, 47 dBm minus 3dB is of course 44dBm OR 25.11 watts.  Getting that out of the way.

So we have a 44dBm signal going into a repeater antenna with 6dB of gain.  So that's 50dBm or 100 watt's ERP if you want to go that route.  ERP is Effective Radiated Power.  Look it up for a better explanation.

 

Now we need to consider path loss.  Path loss is the amount of signal drop between the two antenna's ( repeater transmitting antenna, and your receiving antenna).

The path loss calculator used for these calculations.

https://www.pasternack.com/t-calculator-fspl.aspx?utm_campaign=Power_Combiners&keyword=&gad_source=1&gclid=Cj0KCQiAkKqsBhC3ARIsAEEjuJj7ApOIs_PMeM7_yH6hraxqwaoC9i42tDkgTTaSKcF2Q5qN0GSO0coaAtBGEALw_wcB

Path loss on a 9 mile path for 462.550Mhz is 109dB.

So we take 50dBm and subtract 109dB which leaves up with -59dBm.

Your receiver will open with a tight squelch setting at about -110 to -105dBm signal level. 

So lets add the null to the -59dBm signal level and we get -70dBm.  This is when the signal null is the greatest when the antenna null is turned to face the transmitted signal.

So your radio will open up at -110 to -105dBm and you have a signal level of -70 dBm at the antenna.  Even with another 3dB of signal loss in your coax feeding the receiver, you STILL have a -73dBm signal level hitting your receiver.  To put into contrast the difference between -103 and -73 dBm which is 30dB of course.

If you have a 1 watt transmitter, and you amplify the 1 watt by 30dB, you now have 1000 watts.  It's THAT BIG of a change in signal level.  And your receiver is hearing that much more signal than it needs to in order to open the squelch up and hear the signal. 

So, is your yagi defective?  I am gonna say NO.  And this is the math that proves it's working. 

 

If you ever worndered how the FCC can sit in your neighborhood and gell how much power your CB radio is putting out without needing to walk in and test it with a watt meter, these equations are it.  You measure signal level off of a calibrated antenna into a signal level meter or spectrum analyzer and calculate it back.  Your antenna of course is clearly visible above your house, and it's design will indicate a reasonable gain number to apply to the math to figure out just how much fire you are putting in the wire. 

And mind you, don't take my word for all this.  There are two RF engineers on this board,  One has posted in this thread.  If he chooses to pipe up he can verify this math is correct and why you are hearing the signal off the side of your beam.  And also verify that even if the repeater output was reduced to 1 watt, you would STILL hear it off the side of your beam.  The math don't lie.    I would ask him, if he reads this, that if it's the case, that he bump the answer over on the top left of the post here.  It makes my numbers look a bit better.  And I will continue to do that for him as well. 

Gonna toss this link in here too, gives a better explanation of a yagi and how it effects signal pattern for both receive and transmit. 

 

I follow your ERP calculations using a high gain antenna, so can I use a Yagi with gain of 7.8 dbd considering my 50' of RG8X has a loss of 4.3 db and I keep my Xmtr power at 15 watts and not exceed 50 watts ERP on chs 15 - 22 simplex? 

I calculated  41.7 db (15 watts) - 4.3db + 7.8 dbd = 44.8 db = 30 watts ERP

Edited by WRZN263
Fixing math
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1 hour ago, WRZN263 said:

I follow your ERP calculations using a high gain antenna, so can I use a Yagi with gain of 7.8 dbd considering my 50' of RG8X has a loss of 4.3 db and I keep my Xmtr power at 15 watts and not exceed 50 watts ERP on chs 15 - 22 simplex? 

I calculated  41.7 db (15 watts) - 4.3db + 7.8 dbd = 44.8 db = 30 watts ERP

Yes, but I don't believe that you have an ERP limitation on 15-22 that you are required to meet. 

That being said, it's always a 'best practice' regardless of regulation to only use the minimum amount of power needed to establish a solid communications path with the other party.  Using power levels beyond that can cause interference for other users of the service.  But there is no specific regulation stating that requirement. 

 

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