At 12:06 PM 4/30/98 -0400, DOUG WALLER wrote:
>G/M: SINCE BEAM GAIN VARIES SO WIDELY REL TO FREQ, AS SEEN FROM
>RECENTLY POSTED TABLES HERE, I STARTED PONDERING VARIATION OF GAIN ON
>TRANSMIT VS. RECEIVE.
> IN HIS BOOK "HF ANTS FOR ALL LOCS", MOXON STATES ".....INCREASING
>THE NUMBER OF ELEMENTS FROM TWO, TO THREE OR FOUR WITH CLOSE SPACING HAS
>COMPARATIVELY LITTLE EFFECT ON TRANSMITTING GAIN." AND ".....IN TERMS
>OF TRANSMITTING GAIN, THE LARGER (3&4 ELE) BEAM IS SLIGHTLY DOWN EVEN
>COMPARED WITH THE BEST THAT CAN BE ACHIEVED WITH TWO ELEMENTS,......".
>FINALLY ".....FOUR-ELEMENT DESIGN IS QUITE POOR IN THIS RESPECT(XMIT
>GAIN) SO THAT THERE SEEMS TO BE HERE A PARTICULARLY STRONG CASE FOR THE
>USE OF SEPARATE TUNING UNITS FOR TRANSMISSION AND RECEPTION."
> WOW, IF THIS IS TRUE, I WUD SURE LIKE TO SEE TRANSMIT GAIN FIGURES
>FOR SOME OF THE MORE POPULAR BEAMS!
> IF ANY PARASITIC GAIN EXPERTS CAN SHED MORE LIGHT ON THE ABOVE
>STATEMENTS, I AM HUNGRY TO HEAR IT, HAVING RISKED MY LIFE AND WALLET TO
>GET TO TOWER/BEAM COMBO I PRESENTLY HAVE.
> WILL DEFINITELY ATTEND THE "ANTENNA TECHNOLOGY" FORUM AT DAYTON. WE
>ALL NEED A BIG BREAKTHRU IN DIRECTIONAL GAIN, BOTH XMIT AND RCV.
> 73/DX, DOUG - NX4D
The best succinct, non-mathematical answer I have seen is that yagi gain is
roughly proportional to boom-length. If Moxon means packing 3 or four
elements into the same boomlength, then it makes a certain amount of sense.
On the other hand, I recall one well-known contester saying that he added
an element to a popular 4-element antenna without increasing the
boom-length, in order to broaden the bandwidth of good SWR and gain
performance. Clearly, this is a complex set of tradeoffs.
I have no idea what he means by the need for separate tuning units on
receive and transmit.
A final comment -- while longer booms and more elements may not increase
gain proportional to the effort and expense involved, they definitely aid
front-to-back ratio and narrow the forward beamwidth. Whether these
changes are beneficial in all circumstances is doubtful, but there will be
times (even at HF).
73, Pete Smith N4ZR
In wild, wonderful, fairly rare WEST Virginia
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|