Yagi Antenna Question - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues - primarily having to spend time purging garbage posts from the board. At some point I might start the RF Cafe Forums again if the phpBB software gets better at filtering spam.

Below are the old forum threads, including responses to the original posts.

-- Amateur Radio
-- Anecdotes, Gripes & Humor
-- Antennas
-- CAE, CAD, & Software
-- Circuits & Components
-- Employment & Interviews

-- Miscellany
-- Swap Shop
-- Systems
-- Test & Measurement
-- Webmaster

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

But given answer is 414uW

So what am I doing wrong here?

Thanks

Kirt Blattenberger

Post subject: Re: Yagi Antenna Question

Posted: Sun Jun 12, 2011 5:29 pm

Site Admin

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?

_________________
- Kirt Blattenberger :smt024

RF Cafe Progenitor & Webmaster

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