wire5.art

一个很小和天线计算程序

SUBJECT: Wire antennas - part 5


RHOMBICS

    I've talked about rhombics before and in this posting I 
would like to discuss the rhombic for use in the UHF range. We 
have hashed over rhombics in the HF range and many antenna 
books discuss this antennas.  Most AROs don't have the room on 
their property or do not have property to construct a rhombic 
on.  There seems to be very little on rhombics for UHF work.

    After looking at some plans for a multi-element rhombic on 
70cm, I decided to modify the plans and came up with a 23cm, 12 
element rhomboid. The official name is a dual hexamerous rhomboid 
which is 2X6 rhomboid antenna.  The antenna has twelve elements, 
with a gain of 33dBi, an is no bigger than a TV antenna!  On 
70cm, the antenna is 19-ft long (apprx) and below that I wouldn't
bother unless you have the tower and room. Unless you can mount a 
B-52 to a tower, 2m is out of the question.

    Testing on the 23cm DHR shows a Front to back ratio greater 
than 30:1 with front to side ratios about the same.  This is a true 
cannon. The main lobe is tight at 12 degrees both vertical and 
horizontal. Matching is a bit difficult since micro-strip 
knowledge is a must. Although the antenna is 50 ohms balanced, 
the frequency demands exactness. This involves phasing elements, 
stripline baluns, and hardline feeds. But for the 23 experimenter, 
this antenna packs the punch of a large parabolic reflector and 
in a small package.  I'll be giving the details in the next posting.


 

-WS