# Yagi Antenna Question - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

canuck1
 Post subject: Yagi Antenna Question Posted: Fri Jun 10, 2011 9:20 am
 Lieutenant

Joined: Sun May 22, 2011 6:02 pm
Posts: 2
Not sure if this needs to go into education section or antenna question but here is the question:

Question:
Yagi Antenna Gain = 10dbi
Front to back ratio = 15 db
Distance between Tx and Rx = 15KM
Frequency = 10 MHz
Transmitter ERP = 100KW
Rx feedline loss = 2db

Calculate Signal power supplied to Rx if the antenna is:
a. Pointed directly towards the transmitting antenna
b. Pointed directly away from the transmitting antenna

This is what I've done

I calculated free space path loss using the equation
FSPL(dB) = 20logd + 20logf + 32.45 (where d = distance in KM and F = Frequency in MHz)

FSPL = 23.5 + 20 + 32.45 = 75.95dB

Rx Feedline Loss = 2dB

Total Loss = 77.95

Antenna Gain = 10dBi

Net loss = 77.95 -10 = 66.95dBi

Tx Power = 100 KW

10 Log (Rx Power/ Tx Power) = -66.95dBi

Rx Power = 20.18mW

So what am I doing wrong here?

Thanks

Kirt Blattenberger
 Post subject: Re: Yagi Antenna Question Posted: Sun Jun 12, 2011 5:29 pm

Joined: Sun Aug 03, 2003 2:02 pm
Posts: 451
Location: Erie, PA
Greetings canuck1:

I get just about the same numbers you do (I get 15.96 mW on the first calculation). The 414 uW vs. 20.18 mW is 16.9 dB - very close to the front-to-back ratio. Using my 16 mW vs. 414 uW is 15.9 dB. Methinks the answer was for using the FtB ratio. What say ye?

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canuck1
 Post subject: Re: Yagi Antenna Question Posted: Wed Jun 15, 2011 5:21 pm
 Lieutenant

Joined: Sun May 22, 2011 6:02 pm
Posts: 2
Thanks for getting back, Kirt.
This is an awesome forum.

I was thinking along the same line. Maybe, the given answer was for part B.

This is an old book so I don't know if I will be able to get in touch with the author.

Posted  11/12/2012

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