"Jerry Martes" wrote in message
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"JIMMIE" wrote in message
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On Jun 28, 1:06 pm, "Jerry Martes" wrote:
"JIMMIE" wrote in message

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On Jun 28, 10:36 am, Richard Clark wrote:
On Thu, 28 Jun 2007 06:19:44 -0700, JIMMIE
wrote:

While pondering the design of a broadband antenna that would cover
from 50 to 450 Mhz I considered a log periodic array. This would
serve
my purpose but the size would be inconvienent. Then I had a thought,
Could I remove the elements that are not required for amateur band
coverage? In my case I am only interested in 50, 144,and 432 MHz
bands, Would this allow me to shorten the boom by an amout equal to
the space occupied by the removed elements and have a gain of 7Dbd.
Could I cover all 3 bands with just the elements for 6 and 2m?
Any thoughts on this would be greatly appreciaated.

Hi Jimmie,

What you are then asking for is NOT a broadband antenna; but rather a
multiband antenna, or even three, interlaced single band antennas.
Don't try this at home with a gaussian bundle.

To answer your question more directly, yes you can remove those
elements that do not contribute to the three bands. Unfortunately,
to
maintain its LDPA aspects, you still have to maintain some outliers
for each band. This doesn't necessarily shorten it, because an
LPDA's
length is driven by many other considerations such as bandwidth and
gain, not just simply extremes (yes, that too is part of the mix,
simply a complex part of the mix).

No to your last question. The matter of resonance is still a first
principle. However, the 2M band is loosely related to the 70cM band,
and if you are building an LPDA (and sweep the elements, chevron
style) there may be a chance of it performing there too.

All-in-all, don't expect remarkable gain after having gone through
all
these exceptions.

73's
Richard Clark, KB7QHC

Thanks this is what I was expecting but I think I am going to try it
anyway.
SNIP
Jimmie

Hi Jimmie

Have you figured out which elements to remove and which will remain?

Jerry- Hide quoted text -

- Show quoted text -

In reality it will probably be designed as if it were 2 or 3 single
band lpda antennas. Cebik has a couple of designs like that and I will
just put them on one boom trying them spead out and with the space
removed. For now I am thinking four four element cells for each band.

Im also curious to find out how a swept element 2m antenna will work
on 70cm.


Jimmie

Hi Jimmie

I dont mean to discourage you from studdying LPDA, but, the "LP" aspect
will be completely lost when the antenna is redesigned to operate on only
your 3 bands.
Correct me if I am wrong. It is my uinderstanding that a Log Periodic
Dipole Array is designed to generate a null in the dirrection *away from*
where you want to communicate. That means that you want to minimize the
radiation in the dirrection from the short elements toward the longer
elements. It is also my understanding that the dipoles that do the
majority of the radiating are choosen by their ability to match the Zo of
the line connecting successive dipoles.

The "V" aspect of the LPV is an attempt to take advantage of the fact
that the dipoles will have impedances appropriate for matching the line
when they are 1 1/2 wave long as well as when they are 1/2 wave long.
Each of the V'd, one and a half wave long dipoles will have a radiation
pattern that is far different from the halfwave dipole. But, when the
"V" angle is correct that resultant pattern is appropriate for this
application.

It might be, as Richard Clark has pointed out, that you could build a 6
meter band antenna that outperforms the redisigned LPDA. It seems almost
certain that antennas for the 2 meter and 70 cm bands can made using
*other* design concepts.

Jerry


I absolutely agree, building a 4 element Yagi for each band of interest
would be the most resonable thing to do but I came across a couple of old
LPDA TV antennas and I am itching to do something with them. They are both a
little beat up but firstI am going to try to make repairs to them and sweep
them with my antenna analyzer and graph their impedance.


Jimmie