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: 4 MHZ Antenna bandwidth and < 2:1 vswr  (Read 792 times)
VE4AOD
Member

Posts: 23




Ignore
« on: October 06, 2009, 10:17:25 AM »

Hi. I purchased a HYGAIN VB-25 new and have had nothing but problems. HYGAIN sent me a new 4:1 balun but that doesn't change the fact I cant get 2:1 vswr over more than 2 MHZ b/w. I had another ham review everything (to be safe) and he cant spot the problem either. I have made an RG213 4:1 coax balun 30.5" overall length,short pigtails,and 26.5" of that 30.5" is the coaxial part) Do you fellows think 4 mhz b/w is unrealistic with a 4:1 coax balun? I looked at the manual which said it was optimized for 146, and saw 17.25 DE length. Yet the cutting chart in the manual suggests another length! I am looking into buying a much more expensive and better quality 2M yagi due to the frustration. Thanks for any help as it is much appreciated!
Logged
KB4QAA
Member

Posts: 2490




Ignore
« Reply #1 on: October 06, 2009, 11:36:27 AM »

Huh.  Looks like it ought to meet the specs without too much trouble.  The DE half-lengths do not make sense between the two charts and the dimensions.  Thats really odd.

There can be a lot of ground interaction with swr. Are you able to get any height for testing?  If you can't, then aim the antenna vertically and do your best.

I think something really simple (not that it isn't hard) is the problem.  I recommend disassembling the driven element and match and recheck.  I've never seen a beta match before and the diagram is a bit unclear to me.  Put an ohmeter on the balun and check for shorts. My best guess is that something is getting shorted or missing an insulator.

Good luck!  Bill
Logged
KC9Q
Member

Posts: 49




Ignore
« Reply #2 on: October 09, 2009, 04:50:08 PM »

First the length of your RG-214 coax for the balun should be 25.387". That's the length from shielded end to shielded end. Allowing about 1.5" at each end for a pigtail, that's about an overall length of 28.387". Looking at the manual for the VB-25FM 5 element yagi, the driven element should be in the area of 16 13/16" in length. With those dimensions you should have a 4 MHz bandwidth under 2:1.  Just mount the antenna about 8 ft to 10 ft up to reduce ground effects, and then measure the VSWR.

Have Fun,
Mike
Logged
VE4AOD
Member

Posts: 23




Ignore
« Reply #3 on: October 09, 2009, 05:39:33 PM »

Thanks vy much for the advice!
Logged
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #4 on: October 10, 2009, 06:28:50 AM »

What are you measuring SWR with?
Logged
VE4AOD
Member

Posts: 23




Ignore
« Reply #5 on: October 10, 2009, 07:59:42 AM »

Hi. I am using an AEA 140-525 analyzer. I have also used a DAIWA swr meter to confirm.

I was wondering about the effects of proximity of the antenna to the ground when measuring swr.

I bought the ARRL 2009 handbook/cd and am going to purchase the antenna book as well.

Thanks for the help. I am trying to learn all I can, and find that my reading coupled with asking Elmers
is very beneficial.
Logged
W8JI
Member

Posts: 9296


WWW

Ignore
« Reply #6 on: October 10, 2009, 05:24:15 PM »

I expect it would be very difficult to have a multi-element array have 3% bandwidth at 2:1 SWR. It could only be done if you give up gain for bandwidth.

Let me put this in perspective of an HF antenna....

The bandwidth you want is like building an HF Yagi that covers from below twenty meters to above 20 meters with a low SWR. That's easy with three elements if you give up some gain, much more difficult with four, and nearly impossible with 5 elements. As you add elements it gets worse and worse, unless you give up gain.

This is because the driven element in a Yagi actually "feeds" all the other elements, and they all have a certain effective Q. It is like adding more and more tuned circuits to a filter.

The only way you can make it wider is to tighten up the mutual coupling, or to stagger tune the resonances. This means you have to "detune" the parasitic elements, and that will absolutely cost you gain.

To get enough bandwidth to cover 20 meter CW up to perhaps 14,250 with a 2:1 VSWR, I have four element 20 meter antennas on a 36 foot boom. I can add things and do things to improve the SWR bandwidth, but the end result is I give up F/B or gain. I can compensate the feed, like an OWA antenna does, but it gives up a little gain in the process.

I think the only way you will see the bandwidth you want is to use a log or cross-fire feed, or a log periodic section in the antenna. You might even add  enough loss in the system to get that bandwidth.

The only way i can think of to have optimum gain and the bandwidth you want is with a mattress array.

Somehow you have to give something up for that bandwidth because there is no free lunch. To me, the SWR you have sounds reasonable if the antenna has good gain.

73 Tom
Logged
VE4AOD
Member

Posts: 23




Ignore
« Reply #7 on: October 10, 2009, 06:35:34 PM »

Thanks Tom!
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!