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Comm Antennas

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MJKTuck(at)cs.com
Guest

Posted: Sun Jan 26, 2020 3:32 pm Post subject: Comm Antennas

So I was able to borrow a MFJ-259 tester and hooked up my old 'monopole' 

VHF antenna to measure the resonant frequency of the antenna and the 

SWR. The resonant frequency is the point at which you get the lowest SWR 

on the meter.



I found the resonant frequency at 116.4 but still had a SWR of 2.4 

(ideally you want it close to 1) so this is high.



At our typical frequencies between 118.0 mhz and 138 mhz, SWR ranged 

between 2.7 and 3.6. I thought this rather disappointing although it 

might explain why my transmissions at the upper end (134.80) to ATC are 

rather difficult for them to hear.



I did hook up a Bob Archer antenna which is a dipole and appears pretty 

simple with no apparent scope for adjustment. I was surprised to get a 

resonant frequency at 109.5 mhz with a SWR close to 1. At 118.0 SWR was 

5.8 and at 138 mhz it was 6.5! I don't think it is going to work with 

readings like that.



I guess the question is, what comm antennas have others used that have 

been found to be successful? I'm particularly interested to know the 

best way of providing a suitable ground.



Regards,

Martin Tuck

Europa N152MT

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brian.davies(at)clara.co.
Guest

Posted: Mon Jan 27, 2020 9:39 am Post subject: Comm Antennas

Hi Martin,



I used the standard Europa Dipole in the tail and achieved SWR of 1.25 at the lower frequency, 1.11 mid frequency and 1.51 at top.  This simply involved cutting bits off the copper strip until the best reading was obtained.



Regards



Brian Davies



--

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grahamsingleton(at)btinte
Guest

Posted: Mon Jan 27, 2020 5:05 pm Post subject: Comm Antennas

With remembering that the transmission pattern of the 1/2 wave dipole is much flatter than a quarter whip. You will have much better reception at long range .

Graham



[quote] On 27 Jan 2020, at 17:37, Brian Davies <brian.davies(at)clara.co.uk> wrote:

 

 

 

 Hi Martin,

 

 I used the standard Europa Dipole in the tail and achieved SWR of 1.25 at the lower frequency, 1.11 mid frequency and 1.51 at top.  This simply involved cutting bits off the copper strip until the best reading was obtained.

 

 Regards

 

 Brian Davies

 

 --

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h&jeuropa

Joined: 07 Nov 2006
Posts: 634

Posted: Tue Jan 28, 2020 4:56 am Post subject: Re: Comm Antennas

Hi Martin,



Boy, those are surprising results!  I too used a MFJ analyzer with good results.



I’m assuming when you “measured resonate frequency” you mean that was the frequency with the lowest SWR.  



To start, verify that the frequency calibration of the MFJ is close.  Attach a dummy load (can be 3 150 ohm resistors in parallel or a simple dipole) and sweep across the com frequencies while listening on a hand held or your comm radio.  There is no modulation on the MFJ but you should hear something.



If the MFJ calibration is close, as I expect it is, then you need to tune your antennas.



For the whip on the belly, be sure you have a good ground plane.  Bob Nuckolls  does a pretty good job explaining how a dipole works and what constitutes a ground plane.  Have a look at page 13-11 of AeroElectric Connection book for details.  If you have something similar and there is no corrosion and the connection to the coax is good (assuming you are using RG58 or RG400 coax and the connector is properly attached), then you need to trim your whip.  It should start at about 22 inches long.  Try just taking off ¼" to 3/8 “ and see what difference that makes.  Fortunately your antenna is too long!



I’m really surprised at the Bob Archer antenna.  Did you test it installed or just sitting on a table?  Remember any metal near it will affect it.  This thread from VAF in post 7 has instructions on how to tune a Bob Archer antenna using a MFJ 259!  



http://www.vansairforce.com/community/showthread.php?t=142149&highlight=tuning+bob+archer



It is a bit more complicated since the length will determine the resonant frequency but the gamma matching circuit will determine the SWR.  Although the antenna is new, be sure there is no corrosion where the aluminum bits are riveted together.  



Finally, you can construct a simple dipole per AEC using solid copper house wire (12-14 gauge) and solder RG58 or RG400 coax to it.  Don't worry about a balun or torrids, this is just for testing and education.  Just tape the elements to a piece of wood.  Lead the coax at 90 deg to the elements.  See what the MFJ says about it and try trimming to see the effect.  You might want to do this first to get some confidence in what you are doing:)



I hope some of this helps.  Probably whoever loaned you the MFJ can assist.



Jim Butcher

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MJKTuck(at)cs.com
Guest

Posted: Tue Jan 28, 2020 10:35 am Post subject: Comm Antennas

Thanks Jim, some good information there. I’m suspecting possible corrosion or just bad contact with the ‘whip’ antenna. I also suspect that the aluminum tubing which used to be the static port hook up might be messing with the Bob Archer As it is in very close proximity - might even be touching! I’m also planning to try out an Advanced Aircraft Electronics high gain antenna from Aircraft Spruce which does not require a ground plane and recommended for composite aircraft.



Regards,

Martin



Sent from my iPhone



 	  Quote: 	
		   On Jan 28, 2020, at 12:07 PM, h&amp;jeuropa <butcher43(at)att.net> wrote:

 

 

 

 Hi Martin,

 

 Boy, those are surprising results!  I too used a MFJ analyzer with good results.

 

 I’m assuming when you “measured resonate frequency” you mean that was the frequency with the lowest SWR.  

 

 To start, verify that the frequency calibration of the MFJ is close.  Attach a dummy load (can be 3 150 ohm resistors in parallel or a simple dipole) and sweep across the com frequencies while listening on a hand held or your comm radio.  There is no modulation on the MFJ but you should hear something.

 

 If the MFJ calibration is close, as I expect it is, then you need to tune your antennas.

 

 For the whip on the belly, be sure you have a good ground plane.  Bob Nuckolls  does a pretty good job explaining how a dipole works and what constitutes a ground plane.  Have a look at page 13-11 of AeroElectric Connection book for details.  If you have something similar and there is no corrosion and the connection to the coax is good (assuming you are using RG58 or RG400 coax and the connector is properly attached), then you need to trim your whip.  It should start at about 22 inches long.  Try just taking off ¼" to 3/8 “ and see what difference that makes.  Fortunately your antenna is too long!

 

 I’m really surprised at the Bob Archer antenna.  Did you test it installed or just sitting on a table?  Remember any metal near it will affect it.  This thread from VAF in post 7 has instructions on how to tune a Bob Archer antenna using a MFJ 259!  

 

 http://www.vansairforce.com/community/showthread.php?t=142149&highlight=tuning+bob+archer

 

 It is a bit more complicated since the length will determine the resonant frequency but the gamma matching circuit will determine the SWR.  Although the antenna is new, be sure there is no corrosion where the aluminum bits are riveted together.  

 

 Finally, you can construct a simple dipole per AEC using solid copper house wire (12-14 gauge) and solder RG58 or RG400 coax to it.  Don't worry about a balun or torrids, this is just for testing and education.  Just tape the elements to a piece of wood.  Lead the coax at 90 deg to the elements.  See what the MFJ says about it and try trimming to see the effect.  You might want to do this first to get some confidence in what you are doing:)

 

 I hope some of this helps.  Probably whoever loaned you the MFJ can assist.

 

 Jim Butcher

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=494497#494497

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h&jeuropa

Joined: 07 Nov 2006
Posts: 634

Posted: Tue Jan 28, 2020 12:40 pm Post subject: Re: Comm Antennas

Martin,



I'd like to suggest you make a simple dipole of copper ribbon like the one shown in the build manual annex C.  You can omit the ferrite beads.  Place it so the copper ribbon is vertical with the coax connection near the fuselage joint.  Put it as far forward in the rear as you can, close to the D panel.  The ends of the copper ribbon will wrap on the top and the bottom of the fuselage but most of the radiating section will be vertical.  Be sure there is nothing metallic near or touching it.  Use the MFJ to adjust the length.  



You can purchase the copper ribbon from RST Engineering or ACS.  ACS has a Quickie Kit that appears to have the ribbon, ferrites, coax and connectors.



Jim

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