Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Cushcraft A4S tuning instructions?  (Read 3481 times)
NZ5N
Member

Posts: 774


WWW

Ignore
« on: June 06, 2013, 02:40:47 PM »

Anyone know how to tune an A4S or where to obtain info about this?  The manual just says to "recheck the dimensions" if the antenna has a high SWR on the frequencies where it is supposed to work.  I rechecked several times, all dimensions set to the "center" dimensions shown in the manual (manual gives dimensions for CW, phone, and center).  My A4S is about right on 10m (lowest SWR about 28.7 mhz, should be lower), a little low on 20m (lowest SWR at 13.9 mhz), and way low on 15m (lowest SWR at 20.7 mhz).  Which dimensions should be adjusted to increase the resonant frequency on 20 and 15 and perhaps lower the resonant frequency a bit on 10m?

Thanks and 73, Bill NZ5N
Logged
WB2WIK
Member

Posts: 20611




Ignore
« Reply #1 on: June 06, 2013, 03:39:22 PM »

At what height above ground is the antenna when you're making these measurements?

That determines tuning quite a bit until the antenna is up about a wavelength.
Logged
KC4MOP
Member

Posts: 743




Ignore
« Reply #2 on: June 06, 2013, 05:29:07 PM »

I have owned a couple of Cushcraft products and the dimensions always prove themselves out once it is in the air. 30 feet from the ground is the beginning of a happy Yagi.
There is also another tuning trick.
20M and up, you can make adjustments on the ground with antenna well supported and the reflector one foot from the ground. The antenna pointing straight UP to the sky. You can adjust that way without interaction from the ground.
Logged
K3GM
Member

Posts: 1815




Ignore
« Reply #3 on: June 06, 2013, 06:14:32 PM »

........There is also another tuning trick.
20M and up, you can make adjustments on the ground with antenna well supported and the reflector one foot from the ground. The antenna pointing straight UP to the sky. You can adjust that way without interaction from the ground.
Like this. Although I placed my reflector about 6' from earth.
http://i9.photobucket.com/albums/a96/TwoSevenRight/DSCN1142.jpg
« Last Edit: June 06, 2013, 06:18:44 PM by K3GM » Logged
NZ5N
Member

Posts: 774


WWW

Ignore
« Reply #4 on: June 06, 2013, 06:47:33 PM »

Thanks for he replies.  Yes, I have it pointing staright up, reflector 2' above the ground.  Now, how to make the adjustments?
73, Bill NZ5N
Logged
WX7G
Member

Posts: 6128




Ignore
« Reply #5 on: June 06, 2013, 07:53:01 PM »

I think that it should be left as it is and raised to the final height.
Logged
KC4MOP
Member

Posts: 743




Ignore
« Reply #6 on: June 07, 2013, 03:28:06 AM »

I think that it should be left as it is and raised to the final height.
I agree.
It is going to get really nasty trying to re-work/re-design something that was simply, measure and raise up. The manual is telling you to recheck dimensions and raise it up. You'll go crazy with all of those elements interacting with each other. I assume this was a new purchase?
What height was the antenna when you started analyzing? Were you using the MFJ 259?
The height above ground is really critical.
Logged
WX7G
Member

Posts: 6128




Ignore
« Reply #7 on: June 07, 2013, 05:51:52 AM »

In 1970 I did the "aim the beam straight up" thing to set the tap on a Yagi-Uda gamma match. When raised to the final height the gamma capacitor was then tweaked.

I don't think the "aim the beam straight up" can work well for final tuning, especially not with a triband Yagi-Uda.
Logged
NZ5N
Member

Posts: 774


WWW

Ignore
« Reply #8 on: June 07, 2013, 01:10:06 PM »

As suggested, I put the A4S up into operating position on a 35' tower, but no joy, the SWR was exactly the same as it was when the antenna was pointed straight up.  I can only use it on 10m and on 20m CW, the SWR is too high on 20m above 14.1 mhz (lowest SWR at 13.9 mhz) and on 15m (lowest SWR at 20.7 mhz).  KC4MOP, this is not a new antenna.  Probably the capacitance changed in one or more of the traps.

73, Bill NZ5N
Logged
K3GM
Member

Posts: 1815




Ignore
« Reply #9 on: June 07, 2013, 02:24:38 PM »

Perhaps this comes too late because it sounds like you have disassembled your A4 already.  But anyway, there are 2 sets of traps that look the same, but are different in their resonant frequency.  If you juxtapose them on the assembled antenna, you will suffer degraded performance.  When your A4 was new, the traps had a paper sticker indicating which was which. Why the traps weren't engraved or marked with a permanant sticker is  mystery that only CC can answer. Once mixed up, the traps of the A3 and A4 can only be separated by finding their resonant frequency.  First, the 10 meter traps are the same and you can use them in any position. Be sure the end with the contact screw for the body to element junction is nearest the boom. The 15 meter traps can be identified by the fact that they are "swagged" to a smaller tubing size on the outboard side. There are 2 different types of 15 meter traps. There are two 15 meter traps that are used in the director that have a higher frequency of resonance. That is the only way to tell the difference. Check the traps with a dip meter and that should tell you which are which.  They are very close in frequency so it isn't an easy task.  Another way and undoubtedtly the best way it to attach them with very short leads to an HP network analyzer. Punch up the Smith chart.  Start around 18 Mhz , and crank the frequency up until the readout on the display switches from inductance to capacitance.  Note that frequency.  You will find 2 whose frequency is about 150Kz higher than the others.  Remember, the traps on an assembled antenna resonate at a different frequency than when they're sitting out separately on a bench. So, don't be alarmed if you see them resonate outside (lower) the 15M band.  Good luck to you.

As a side note.  Years ago in the golden years of Cushcraft, they use to offer a service to those who had mixed up the traps.  They had a bench fixture they would drop them onto to quickly measure the resonant frequency of the traps.  Again, it would have been so much easier to permanently identify them inthe first place.....
« Last Edit: June 07, 2013, 02:33:52 PM by K3GM » Logged
NZ5N
Member

Posts: 774


WWW

Ignore
« Reply #10 on: June 07, 2013, 03:13:32 PM »

Thanks, the antenna is still on the tower, and it will stay there for some time until I can get help to take it down.  The traps do have labels and I feel confident they were installed in the correct positions.

73, Bill NZ5N
Logged
NZ5N
Member

Posts: 774


WWW

Ignore
« Reply #11 on: June 07, 2013, 04:34:05 PM »

What makes this confusing to me is that the low SWR point is higher than it should be on 10m but lower than it should be on 15m and 20m.  And there is almost no room for adjustment between the 15m and 10m traps.  If I shorten the elements between the 10m traps, that should raise the frequency and 10 and 15 but make it worse on 10m.  If I shorten the end of the antenna, that should raise the frequency on 20m but should have little or no effect on 15m or 10m. 

73, Bill NZ5N
Logged
KC4MOP
Member

Posts: 743




Ignore
« Reply #12 on: June 07, 2013, 06:48:34 PM »

Bill
I owe you an apology OM. I hope my replies did not convince you to put the antenna on the tower and it is useless on the bands you mentioned. I did not know it was used and maybe there are some traps in the wrong place, the direction of the trap was important also. OR the previous owner had unresolved problems and just passed them on to you.
Your previous replies here do not look promising on the problems you are having with the antenna. A4S is supposed to be a very good and stable product.
Fred
Logged
VA1CQ
Member

Posts: 70


WWW

Ignore
« Reply #13 on: June 08, 2013, 02:36:54 PM »

I found that if the element dimensions are all correct, the traps are in the correct locations and the traps are still healthy, then my A4S mounted at about 9 feet above ground on a short self-standing tower tuned correctly. The land in the forward direction of the antenna did slope down which may have helped reduce interaction.
Logged
KQ9J
Member

Posts: 35


WWW

Ignore
« Reply #14 on: June 10, 2013, 04:52:05 AM »

I had put one of the 10 meter driven element traps in backwards when I reassembled my A3, and it caused no end of troubles. Easy mistake to make. And no, there isn't much room for adjustment between the traps, but it doesn't take much.  You'll get it.  Smiley
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!