[TowerTalk] Antenna gain

Jim Miller JimMiller at STL-OnLine.Net
Thu Jun 29 18:01:25 EDT 2006

His parents are very supportive and are glad he has become interested in a hobby where he is receiving the help and assistance and information from other hams and not doing something else he could be.  He got interested, read about it and got his license almost entirely on his own and is now asking a lot of questions.  His mother is just thinking and asking questions.  I was in the process of preparing a presentation on RF Safety to the club and had read the OET65 some time ago and was just going over the Supplement B again when this whole thing came up and Ryan came to our club.  I showed her OET65 and Sup B and OET56 and the URLs where she could get copies of them and my scratch sheet from my station with references to pages and figures where the figures came from.  I explained the effect of variation of frequencies, power levels, and antenna gain and the definition of and difference between controlled and uncontrolled and she is now comfortable with his intended installation.  I also explained the better installation of getting it up in the air if possible.  She is going to give the task of research and evaluation to Ryan and monitor the process but with my explanation to her, is now comforted that there will not be a problem and knows the safe distances. for his equipment.  Nice to run into intelligent, questioning , non-confrontational, supportive parents dedicated to their kids once in a while.

Try this:  ask the next ham you see if he/she has any documentation (scratch paper or ANYTHING) showing they have done or do not need to do the evaluation or even thought about it.  Most likely you will get a "what"?  Don't be surprised.  I know, you signed or will have to sign when you get or renew your license that you have read and understand the regulations.

Excuse the soap box please,

Tnx es 73, de Jim KG0KP

  ----- Original Message ----- 
  From: Jim Lux 
  To: Jim Miller ; TOWERTALK Reflector 
  Sent: Thursday, June 29, 2006 12:14 PM
  Subject: Re: [TowerTalk] Antenna gain

  At 08:29 AM 6/29/2006, Jim Miller wrote:

    What is the gain of a J-Pole, dBi or dBd?  About 3 dBi?  More?  Less?

  A J pole is basically an endfed dipole, so I'd expect the gain to be in the 2-3 dBi range..

    Talk about stepping DOWN from towers and beams.  BUT this IS an antenna question.

    A new ham who just had his 12th birthday at field day and received a 2 meter radio now has a J-Pole antenna given to him until he gets something better.  The question about RF exposure comes from his parents.

  How much power is he running? I assume he's done the  analysis  (aka "routine evaluation") laid out in the OET Bulletin 65B?  (http://www.fcc.gov/oet/rfsafety).  Following the stuff in the bulletin (such as doing the worksheet in the back) will go a lot towards assuaging parental concerns: more so than a casual "well, it looks ok to me".

  In particular, take a look at Table 4b on page 24 (page 29 in the pdf).  For an FM rig, PEP = rig output power.
  Looks like for 50 Watts, and a 3 dBi antenna, it's 1.4 or 3.2 meters, depending on controlled or uncontrolled. 
  Make sure he understands the difference between controlled and uncontrolled exposures. You only get to use controlled if everyone is aware of the exposure, etc..  This may or may not be the case for family members, especially where the control op is young. {i.e. there's a big difference between me (as dad) operating the radio, being aware of who's in the house, where they are, what the exposure rules are, etc.; and my 12 year old daughter, who may get concentrated on radio'ing, and ignoring where her pesky 9 year old sister is)

  Those numbers are based on a farfield approximation. Lest you think that "hey, my duty factor is only 20%, so I can cut those numbers by a factor of sqrt(5)", you've got to start thinking about the near field.  As Balzano (1981) says: "In the close proximity of sources, the safety critera are no longer valid because the value of |E|^2 is not directly related to power flow, but, rather, to the level of electric energy stored around the radiator."   

  The upshot is that unless you're actually measuring the field, you should use conservatism and lots of worst case assumptions.  Don't bolt that jpole to the table the radio is on.     Put it up on the roof where people will be 10 ft away.

  In the 2 m band, you're looking at a field strength limit of 61.4 V/m (controlled) or 27.5V/m (uncontrolled). Compare this to measurements made by the FCC/EPA for a "
Whip on auto roof" at 146.5MHz with
     22-75 V/m at 2 m from
     15-30 V/m    In
        90 V/m    in
Rear seat  

This is for an antenna that's nominally about 1 dBi. That jpole is higher gain: more energy stored in the near field and higher power densities in some directions in the far field. Note that even 2 m away, you're still above the uncontrolled exposure limit (granted, nobody is going to talk for 6 (or 30) minutes straight, but....)

  Jim Lux, W6RMK 

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