46179 Sep 9 09:27 newyagi.Z (hpgl, pcl) 839984 Sep 9 09:27 newyagi.pcl 55256 Sep 9 12:09 oldyagi.Z (hpgl, pcl) 705518 Sep 9 12:34 oldyagi.pclThese are PCL files (native language for HP laserjet printers), compressed (UNIX compress *.Z) files and *.hpgl is an HPGL file (for those that don't have a laser printer) of 4.2 lambda boomlength ~900 MHz yagi antennas. They were designed by n6gn using the NBS data summarized in Ham Radio Magazine in August 1977 by Joe Reisert, w1jr.
The files oldyagi* are the first design which, ended up centered at about 850 MHz or so. The text says it's for 905 but the actual center of the gain bandwidth is more like 850 MHz. This may have non-amateur uses. I don't know nor can I comment on the legality of any such uses. The second antenna, newyagi, was moved up about 5% to better center it on the 902-928 amateur/ISM/ (junk/computer CB) band. This is the antenna which has been built the most times by the most people in this area for use with the higher speed digital radios.
The actual bandwidth of these antennas may be considerably greater than might be expected. Long Yagi arrays are often thought to be very narrowband structures however the match bandwidth is generally what limits performance rather than the fundamental antenna structure. Network analyzer measurements made of two identical antennas in each other's far field have shown 50-100 MHz of available bandwidth. The display of this measurement looks a good deal like that of a multi-section bandpass filter, being fairly flat across the "passband" and dropping pretty suddenly at the ends. The high side falls off somewhat more quickly than the low frequency edge of the antenna. Typical match bandwidths may be only 10-20 MHz, depending upon the feed method used.
Many of the second type antennas have been built by a variety of amateurs with equivalent performance. Measurements of these antennas reveal the expected gain/directivity. Forward gain in the region of 14 dBd may be achieved. To get this, it will be important to mount the antenna many wavelengths away from ground and structures. >20' of clearance should be adequate.
Achieving good match over a large fraction of the gain bandwidth may be a challenge. The folded dipole driven element is relatively good in this respect but 10% match bandwidth may be unrealistic. Even taking what you get, two of these antennas back-back indicate fairly flat performance over a pretty wide chunk of spectrum. Mismatch loss probably isn't generally as big a deal as mounting them in the clear and suitably removed from ground and obstructions. I'd expect most modern transmitters to tolerate the load they present pretty well.
Be sure to cover the shiny new brass elements with a durable coating (most paints seem OK) so that the environment won't cause deterioration of the element surface. Such deterioration can rapidly increase loss and degrade the antenna's efficiency. This kind of degradation is particularly insidious because match and directivity aren't affected, the antenna just ceases to provide the expected gain.
This is a photo of a completed antenna mounted vertically polarized in front of a guyed mast. The vertical line going through the picture right behind the reflector is part of a different antenna in the background.
Here is a photo of the folded dipole feed and balun. The thinner through-the-boom portion of the driven element can be seen going through the wider portion of the feed and the balun can be seen parallel to the boom.
