[amsat-bb] Re: Re HELIX REFLECTOR?

John Belstner jbelstner at yahoo.com
Sat Jun 5 17:33:36 PDT 2010

```Hi Steve,

To avoid giving you bad info, I went back and looked at some of the modeling I did a few years ago to refresh my memory and realized that was done on crossed yagis and not helixes.  Recently I did some helix modeling and took some field measurements on a terrestrial sectored antenna application and found that I could not get the F/B any greater than 20 dB without a backplane that was at least 1 wavelength in diameter and with a forward-flared rim at the end (like the lip of a screw top).

Since you asked the question, however, I went back and looked at the model again.  Perhaps the word "adversely" was a bit of an exaggeration.  In the vertical orientation (at 437 MHz), the forward gain of the main lobe didn't change much between 1 and 5 wavelengths above ground; what did change was the depth of the off axis nulls.  More pronounced (of course) is during the transition from vertical to horizontal, the ground reflections raise the pointing angle of the main lobe to as high as 15 degrees off axis at horizontal when the antenna is as low as 1 wavelength above ground.

Bottom line, 1 wavelength I guess.  But not so much because of a degradation in forward gain but because the angle of the main lobe is off axis.

Hope this is helpful and not just a rambling.

John

On Jun 5, 2010, at 2:10 PM, STeve Andre' wrote:

> john, whats a good rule for height of the antennas to avoid large
> back lobes reflecting, ie how many wavelengths up should the
> antennas be?  Thanks...
>
> --STeve Andre'
> wb8wsf  en82
>
> On Saturday 05 June 2010 17:07:15 John Belstner wrote:
>> Just another \$0.02 to add.
>>
>> You will find that the size and shape of the reflector will not affect the
>> forward gain as much as it does the F/B ratio.  It depends on what is
>> important to you and (of course) how high you are above the ground.  Even
>> for satellite operation pointing up, large back lobes reflecting off the
>> ground can adversely affect the forward pattern when the antenna is mounted
>> only 6-8 feet above ground.
>>
>> On Jun 4, 2010, at 10:13 PM, Clare Fowler wrote:
>>> To add to the discussion the July/Aug 2007 Amsat Journal has an article
>>> covering some gain comparisonmeasurements I made between four
>>> 13 turn (2.88 wavelengths) 13cm antennas  with different square solid
>>> aluminum reflectors.
>>> The sizes were 0.56 wavelengths, 0.84 wavelengths, 1.0 wavelength and 1.4
>>> wavelengths.
>>> There was no difference between the 0.84, 1.0 and 1.4 wavelengths but the
>>> antenna with the0.56 wavelength reflector had 1.5 db less gain.
>>>
>>> However for my 70cm helix antennas I followed the Satellite Handbook
>>> minimum
>>> size of 0.6 wavelengthsor slightly over 16 inches. I used 1/2 inch
>>> hardware cloth
>>> These antennas have performed well however it appears that they would be
>>> a bit better with a somewhat larger reflector.
>>>
>>> A brief description and picture of the 70 cm reflector is in the
>>> November/December 2005 Amsat Journal article on
>>> The Development of a Quarter Wave Match for helical antennas.
>>>
>>> Clare  VE3NPC
>>>
>>>>> Hi All:
>>>>> I am rebuilding a 440 MHZ Helix that I built several years ago . It
>>>>> worked very well, but I would like to reduce the size of the reflector
>>>>> to a more manageable size than I had before. The only reference to
>>>>> reflector size I can find is, "minimum 20" ". I may be looking in the
>>>>> wrong places. I would appreciate it, if someone would steer me in the
>>>>> right direction. Thanks,
>>>>> Pete, K1HZU
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