G0KSC - Simple to build, High Performance Yagi and Quad Antennas - Home of the LFA Yagi

Published 22/8/2014

A Super-quiet 5 element Yagi for 73Mhz to 74.6Mhz intended for Radio Astronomy use

Model: 5-73-RA - LFA Yagi

Download the EZNEC files for this antenna HERE

If you wish to buy this antenna ready-built, find it HERE

The LFA Yagi is designed to be very low noise and thus, allow the user to receive weak signals that could otherwise not be worked. As a result, performance with modes such as MS and EME is exceptional.

Note this is wideband and thus stable in wet and icy weather too!

The only place you can buy this antenna is InnovAntennas no other company has permission to sell 73MHz LFA Yagis!

Performance Plots

Note the very fast drop off to high suppression either side of the antenna from the 45 degree line backwards. This is key in ensuring you receive no unwanted noise (from directions other than which your antenna is facing).

Azimuth

This super-clean antenna when placed 10m above ground is a serious performer.

A nice flat, SWR curve as I am sure you have come to expect from me!

Performance

Peak Gain: 10.67dBi

Peak F/B: 29.92dB

Power Rating: 4kw

SWR: Below 1.1.1 from 73.000MHz to 74.60MHz

Boom Length: 3m

Notes: view the rest of the G0KSC site for building tips and hints if you are building a Yagi for the first time. Do ask if you have any questions at all.

So here are the details for this one if you want to go build it yourself.

Boom positions in mm, done this way as mm mean more precise measurements!:

boom positions start at 30mm to allow for the insulator to be fitted to the boom.

Ref: 30mm

DE: 244mm (this is the drive part of the loop See notes below)

DE2: 621mm

D1: 949mm

D2: 1833mm

D3: 3000mm

Element Lengths - (All parasitic elements 1/4'' diameter with the exception of the driven loop which is 1/2'' and has 3/8'' folded dipole ends). See note below on potential correction factors and tuning.

Ref: 2022mm

DE: 1644mm (This is the length the 1/2'' sections need to be cut to)

DE2: 1644mm (see notes below on loop preparation)

D1: 1889mm

D2: 1812mm

D3: 1736mm (this is correct, D3 bigger than D2)

Driven element Notes:

With a folded dipole/LFA loop this design is direct feed, 50Ω and needs no matching device
While you will read elsewhere to the contrary, a choke or balun (1:1) IS required see here
The above listed dipole length has had 50mm removed to achieve the length required for the straight sections in the diagram below and to account for the radius of the trombone-style end sections of the loop.
This is required to allow for correction due to the curves in the loop-ends, the step-down in loop-end diameter and correction due to the loop being earthed opposite the feed point
A tool to bend loop ends to exactly the right size required can be purchased here
It is best to have absolute minimum contact with the drive element itself so use the shortest/smallest form of insulators possible. Avoid feed point enclosures where possible and only use coaxial cable joins that maintain impedance.
Once the loop is together, measure its complete width to the above mentioned dipole length plus 30mm and adjust for best SWR from that starting point
The overall height of the folded dipole can vary if you wish it, but the length of the folded dipole will change. For example, if you change loop height from 46mm to 40mm, the width if the FD will need to be longer then the above stated in order to achieve a perfect SWR. The opposite applies if you increase loop height. The construction of the loop allows for easy SWR adjustment be means of a Trombone type adjustment. See here for more details

Antenna Layout using recommended Stauff clamps and 1.25'' square boom. They are held to the boom by 6mm (M6) bolts. it is recommended that stainless steel components are used throughout.

Download the large-layout version of this diagram HERE

Pitfalls and Cliffs

If you have any questions or want an antenna of a different size, Email me.

There is NO CORRECTION added to any element lengths. If you follow my mechanical construction guidelines, nothing will need to be added at all. DO NOT use any insulators where bolts pass through the boom and element without establishing correction factors, your antenna elements will need to be lengthened should you chose to go down this route.

Any questions just ask. Within this next few weeks, I will add this antenna to as new category and will add another version.

COMMERICALLY AVAILABLE FROM INNOVANTENNAS. NO OTHER COMPANY HAS COMMERCIAL USE RIGHTS OF ANY KIND