SWR issue on DC grounded antenna.

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ZS1JDT

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Hi guy's! New to this forum. From my Username, figure I'm a Ham radio operator. I'm no new-be to antennas and radios... very good with those... till today! As I'm going to be a radio / vehicle Marshall at a 4x4 event, I fitted a 29 Mhz radio to my car. I got with the radio a 29 Mhz Magnetic mount antenna... SWR 1.8 or almost 2. horrible... removed my 2 meter antenna from the roof... removed the magnet and mounted the antenna directly on the roof.

I know its a DC ground antenna as you get that typical "short" testing the feed line on the PL259. Without the coil and antenna and testing on the base, perfect. So the PL259 and base is perfect... BUT I still have the high SWR reading... I like under 1.5!

Obviously I started with the whip... going longer and/or shorter. Guess what... the lowest reading I get is 1.8!
Its a decent REVEX SWR meter 1.8 -200 Mhz that I use all the time. If it was possible to "strip" the base, without breaking it, I could have just removed the 2N 130K component... and make the antenna Non DC grounded... or just regular.

In short... Why cant I get that SWR 1:2:1 ? Maybe is a CB antenna? Whip should then just go shorter for 29 Mhz?
 

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I agree with all the metal clutter possibly making a muck of the readings... but really, an 1.5 to 1 **is not all that bad- what does that translate to-- a <4% reflected power loss ?
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I'd (personally) could live nicely with that.
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Lauri :)
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** I read what was written as "High".. "under 1.5"------- is that correct ? .........if it is under 1.5, well and good...but even 1.8 isn't earth shattering. :)
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prcguy

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The antenna looks like a common 27MHz CB antenna, have you shortened the whip to see if it improves the match? If the antenna was originally tuned to 27Mhz you would have to remove about 6" to work near the 29Mhz range, but do it in small increments to avoid mistakes.

The antenna is a bit close to the window but you should still be able to get a good match. To test for this you can attach several feet of wire to the grounded base of the antenna and lay it down across the window to increase the ground plane in that direction. If you can achieve a good match with the wire but not without it then the mounting location would be the limiting factor.
 

ZS1JDT

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The antenna looks like a common 27MHz CB antenna, have you shortened the whip to see if it improves the match? If the antenna was originally tuned to 27Mhz you would have to remove about 6" to work near the 29Mhz range, but do it in small increments to avoid mistakes.
Thanks for all the replies... I should have mentioned I did trim outside of the carport (metal roof) with a old whip. A google search showed that is a CB antenna... 27 Mhz and for 29 needs to go a lot shorter... I haven't gone as far as 6 inches shorter...

The REVEX show that radio unit power is "tuned" up to almost 10 Watts.... AM CB sets should be all 4 Watts AM and 12 Watts SSB.... so I'm thinking now... that loading coil is factory made and sealed for CB specification. Maybe by adjusting the power output back to 4 Watts.... that should surly make a difference on the SWR reading?

Now I need to find a diagram, for the equivalent of this radio... that is marked in my country as "Tedelex" TE 3000 and in other countries under some other name.... I sure the radio finals will survive short transmissions...but in my heart... never operated on a SWR higher that 1.5!
 

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I suggest you trim that whip very carefully ! - in fact, may I suggest you substitute a length of stiff wire in its stead ?.. a piece of coat hanger is what the guys at my (former work) radio shop always used. They had quite a collection of them for just that purpose... :)
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Lauri :)

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Hi Ipfd.... ;)
 

ladn

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Good advice Lauri. I've done the same thing. In addition to wire coat hangers, I've had good results with lengths of the wire used to support drop ceilings. It's inexpensive, easy to cut and comes in about 48" sections.
 

bharvey2

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I suggest you trim that whip very carefully ! - in fact, may I suggest you substitute a length of stiff wire in its stead ?.. a piece of coat hanger is what the guys at my (former work) radio shop always used. They had quite a collection of them for just that purpose... :)
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I'll second the coat hanger test method. If I think I'm going to need to drastically change the length of a whip, I substitute another piece of wire and hack it up first before I sacrifice my "dress" whip to the cutters.
 

prcguy

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In your case, SWR is a measurement of the "antenna system" which includes the antenna over its ground plane and the feedline. Power level has nothing to do with the results of the measurement except you do have to calibrate the meter if you change power levels. As someone mentioned you could remove the spring and screw the whip and its adapter into the coil which will temporary shorten the whip for measuring SWR.

Thanks for all the replies... I should have mentioned I did trim outside of the carport (metal roof) with a old whip. A google search showed that is a CB antenna... 27 Mhz and for 29 needs to go a lot shorter... I haven't gone as far as 6 inches shorter...

The REVEX show that radio unit power is "tuned" up to almost 10 Watts.... AM CB sets should be all 4 Watts AM and 12 Watts SSB.... so I'm thinking now... that loading coil is factory made and sealed for CB specification. Maybe by adjusting the power output back to 4 Watts.... that should surly make a difference on the SWR reading?

Now I need to find a diagram, for the equivalent of this radio... that is marked in my country as "Tedelex" TE 3000 and in other countries under some other name.... I sure the radio finals will survive short transmissions...but in my heart... never operated on a SWR higher that 1.5!
 

nanZor

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Keep in mind that a quarter-wave vertical over a good horizontal ground is actually 35 ohm. Thus the 1.5:1 swr when measured using a 50 ohm transmission line, AND that measurement is done at an electrical half-wave away from the feedpoint. (typically taking the velocity-factor of the cable into account).

If you don't measure at an electrical half-wave, then your measurements will be off. Or real-world, a reaaally small jumper at the feedpoint, with you and surrounding junk out of the way, and perhaps a pair of binoculars while you test. :)

I'm most familiar with this in the HF world, where a poor lossy ground will actually present a 50 ohm impedance. Then, as you improve the ground with more radials, the swr rises beyond 1:1 to omg, 1.5:1

That actually means you are making progress for a horizontal ground system which wants to be 35 ohm!

But in many cases, I've seen the op get so concerned about the 1.5:1 after improving the ground, that they *detune* the antenna back to 1:1.

The fastest way to do this very efficiently is to not use SWR as an indicator of resonance, but use an antenna analyzer, and watch for minimum reactance.
 

prcguy

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The particular antenna the OP is using has a loading coil and a shunt coil for matching, so it should be around 50 ohms at resonance with a typical car roof under it.

Keep in mind that a quarter-wave vertical over a good horizontal ground is actually 35 ohm. Thus the 1.5:1 swr when measured using a 50 ohm transmission line, AND that measurement is done at an electrical half-wave away from the feedpoint. (typically taking the velocity-factor of the cable into account).

If you don't measure at an electrical half-wave, then your measurements will be off. Or real-world, a reaaally small jumper at the feedpoint, with you and surrounding junk out of the way, and perhaps a pair of binoculars while you test. :)

I'm most familiar with this in the HF world, where a poor lossy ground will actually present a 50 ohm impedance. Then, as you improve the ground with more radials, the swr rises beyond 1:1 to omg, 1.5:1

That actually means you are making progress for a horizontal ground system which wants to be 35 ohm!

But in many cases, I've seen the op get so concerned about the 1.5:1 after improving the ground, that they *detune* the antenna back to 1:1.

The fastest way to do this very efficiently is to not use SWR as an indicator of resonance, but use an antenna analyzer, and watch for minimum reactance.
 

Spec

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Where the spike meets the spring is a hex screw. Using this hex and moving the spike up and down as needed is often all you will need on that particular base load ant. IF and only IF you absolutely decide to clip the spike use a template such as a number 12 piece of wire or a old coat hanger to get the exact length.
 

ZS1JDT

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The fastest way to do this very efficiently is to not use SWR as an indicator of resonance, but use an antenna analyzer, and watch for minimum reactance.
I just wish I had an antenna analyzer! That is almost essential in any Ham shack! I went back to the 4x4 club and got a broken antenna base... also a DC Ground "coil" section and stripped it to create a useful diagram.

Yesterday afternoon... messed up my 5/8, 2 meter antenna... Haha! Placed my 2 meter base on the magnet... use a section of fishing rod... Glued the 2 m antenna whip on that... measured the whip length... using the Ham 1/4 wave length formula in meters (not inches, as SA uses metric), I wound a coil around the fishing rod section to get approx 2,75 meters... total, including whip.

First SWR test... SWR 1:2 to 1! My home made will work for the event! I don't wanna give up to solve the issue on the Power Max antenna! I want those answers and I'll eventually find the solution. My gut feeling tells me, the reason why that antenna doesn't want to trim down... is that 2N 130K component or whatever equivalent that is in that base. Have a look.. and you are allowed to LOL on my home made as it's really ugly! Taped up... the spring the works... but the antenna is working perfect!

The second pic I created showing what going on inside a DC Ground antenna. If I can only strip that Power Max to get inside...in such a way... to reassemble it to have strength again on the plastic section... I'll get it to trim!
Thanks for the answer on the power output on the radio! That was useful info!
 

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ZS1JDT

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Problem Solved! I feel like an idiot, but learned something!
I watched few videos... then saw this one... watch it and all will come together!
https://www.youtube.com/watch?v=RndU1pc0br0
Then I figured out what's going on! I tested a few things and made this video! Now you can see what I look like!
https://youtu.be/8WRFJfse0w8
While the video was uploading, I changed the unmarked feed-line to RG-58 C/U.
A few minutes back... Trimmed about an inch and a half off the whip... SWR 1:2 to 1!
The co-ax cable looked OK, but at closer inspection, ever so slightly thicker and the outer plastic a bit more rough compared to the RG 58 C/U.
I hope this will also be useful for someone in the future that experience the same problem!
 
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