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Author Topic: HF mobile (15 meter band), SWR will not go below 2.5  (Read 1811 times)
HS0ZIB
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Posts: 410




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« on: July 02, 2011, 05:19:57 AM »

I have installed an NGC Co Ltd 15 meter band HF rig in my 4x4.  (Decent mobile rigs are very scarce in HS land, so I'm lucky to have even a 20-30 year old rig!).

The antenna is a homebrew helical-wound antenna, using a fibreglass former, about 1 meter in total length and mounted via a commercial antenna mount onto the metal roof rack bars that run each side of the vehicle.

Using a miniVNA, I was able to adjust the length of the helical wire/spacing until the SWR was at a minimum of about 2.4 at 21200 KHz.

That SWR was achieved without any grounding of the antenna base etc, but after I installed a short, sturdy bonding strap between the antenna base and the vehicle roof (not the roof rails), the SWR increased to about 2.5, although the resonant frequency of the ax did not change.

Although I am led to believe that an HF mobile antenna has a lowish impedance, (less than 50 ohms), my VNA indicates a |z| value of about 100 ohms at 21200 KHz.

During my tests, I could receive a station from south China (about 2,500km away), so that looked encouraging, (although he did not respond to my call).

Any suggestions as to how I can improve my HF mobile set-up?  I must homebrew my antenna, due to parcel length restrictions from the USA.  Anyway, I think my existing ax is reasonably ok, but I'm concerned about the highish SWR and the fact that it increased when I grounded the antenna base

Simon


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W8JX
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« Reply #1 on: July 02, 2011, 05:32:54 AM »

Try using a 50 to 100pf silver mica cap between base of antenna and ground. I used this trick with a hamstick years ago to get best match. At lower freqs you increase cap value. I did not use a tuner back then and wanted best possible match.
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HS0ZIB
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Posts: 410




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« Reply #2 on: July 02, 2011, 06:47:33 AM »

Yes, I read about the use of a shunt capacitor.  Since the rig only puts out 10 watts, it should not be difficult to implement this.  My only issue is that I will have to cut into the insulation of the antenna mount/cable, so that I can attach the capacitor

Simon
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W5DXP
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« Reply #3 on: July 02, 2011, 07:15:48 AM »

The antenna is a homebrew helical-wound antenna, using a fibreglass former, about 1 meter in total length

Is this a single-band antenna, i.e. is it electrically 1/4 wavelength long? If so, a shunt cap at the base should fix the problem. Shortened loaded monopoles tend to have a resonant feedpoint impedance less than 50 ohms. For instance, a -j50 ohm shunt capacitor will twist a feedpoint impedance of 25+j25 ohms to a perfect 50 ohms. I have found that a short stinger on the top of a helical antenna will make it better-behaved and more predictable.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W8JX
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Posts: 5492




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« Reply #4 on: July 02, 2011, 07:26:35 AM »

Yes, I read about the use of a shunt capacitor.  Since the rig only puts out 10 watts, it should not be difficult to implement this.  My only issue is that I will have to cut into the insulation of the antenna mount/cable, so that I can attach the capacitor

Simon


Can you tap into base of antenna itself on side of it to attach a shunt cap? If antenna screws into mount, you could use a terminal lug between antenna and screw mount for a attach point for antenna side of shunt cap.
« Last Edit: July 02, 2011, 07:55:03 AM by W8JX » Logged

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WB6BYU
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« Reply #5 on: July 02, 2011, 08:11:27 AM »

Quote from: HS0ZIB

Although I am led to believe that an HF mobile antenna has a lowish impedance, (less than 50 ohms), my VNA indicates a |z| value of about 100 ohms at 21200 KHz.



Hi Simon,

Can you measure the R value?  It is quite possible to have R around 20 ohms and |z| around 100, depending
on the reactance.  If the resistive value really is that high, you must have a LOT of ground loss.

Also, where are you taking the measurement with the VNA?  Unless it is RIGHT AT the feedpoint, you will
need to correct for the coax length.  If you are measuring at the rig end of the coax, you could easily
see R = 100 when the impedance at the antenna is 25 ohms if the coax is a multiple of 1/4 long.

Certainly anything you can do add capacitance or a stinger to the top of the helix will help to make it
more stable and raise the feedpoint impedance.  Even without it, I suspect you will get the best match
with most of the wire bunched towards the upper end of the antenna, though that will require more
wire for resonance (possibly as much as 3/4 wavelengths.)
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HS0ZIB
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Posts: 410




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« Reply #6 on: July 02, 2011, 06:19:57 PM »

I've edited this post after some further experimentation.

I found a 47pF capacitor in my junk box and was able to solder it directly across the antenna base connector.  My VNA then indicated a low SWR of 1.5!

I decided to rewind my helical, with most of the coil at the higher end of the antenna.  This of course, changed the resonant frequency and so I had to adjust the wire length, spacing it out as I wound closer to the base connector.

The final result?  A very good 1.1 - 1.2 SWR at 21200 KHz, with a bandwidth of < 2.0 SWR of 440KHz.

I'm very happy with the measured response of my antenna - the proof will be if I can make the contacts!

Simon
« Last Edit: July 02, 2011, 08:55:37 PM by HS0ZIB » Logged
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