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Author Topic: Tunning 40m Antenna  (Read 9724 times)
M3RLQ
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« on: October 11, 2013, 10:41:58 AM »

I have just purchased a 40m antenna and have a little trouble with the SWR the higher up the band I go.

With the antenna whip set in it's default position (so as you can't lower the whip any shorter), the SWR is great from about 7.045 - 7.100

When I extend the whip a little the SWR starts to increase, so I bring it back again! Now I have brought the whip back again and the SWR has reduced slightly but I want to get to the top of the band limit of 7.200

Now i'm thinking, to get the SWR to reduce at this near frequency I would have to cut a small amount off the whip! Is this normal practice?

It seems to me the higher the frequency the shorter the antenna and shorter the frequency longer the antenna, is this correct?

I just don't want to cut the antenna and ruin it all together.

Any suggestion would be very welcome and by the way I do use the ATU for this!

M3RLQ 
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M6GOM
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« Reply #1 on: October 11, 2013, 11:17:38 AM »

Quote
It seems to me the higher the frequency the shorter the antenna and shorter the frequency longer the antenna, is this correct?
Yes. If you think back to your foundation lessons, the lower the frequency, the longer the antenna. The higher the frequency the shorter the antenna. So a simple dipole antenna for 7MHz will be twice as long as one for 14MHz.

If it is a hamstick type the problem could be the whip is too long so when you have it in its lowest position its actually going inside the coil and screwing up the inductance, sometimes shorting it out and causing all kinds of problems.

One way to test for this is to get some stiff wire and make it just an inch or two longer than the bit of the whip that sticks out when you've got it tuned on 7.045 - 7.100. Replace the whip with that and try lowering it. Because its not as long as the whip it won't start protruding into the middle of the coiled section and causing problems.
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M3RLQ
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« Reply #2 on: October 11, 2013, 12:10:58 PM »

Appreciate your reply,

The antenna is a Diamond HF40CL and comes in two sections. When the whip section is inserted into the center coil section it goes in about inch and a half to 2 inches then hits a dead end stop (doesn't slide into the coil at all).

Been about 11 years since I did the Foundation, memory needs refreshing!
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KQ6Q
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« Reply #3 on: October 11, 2013, 09:52:00 PM »

Try doing the measuring without using the ATU - just the SWR meter in your rig. Optimize the SWR without the ATU, then use the ATU.
If the ATU is trying to adjust the SWR, the reactance it's adding is changing the apparent resonant frequency.
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M3RLQ
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« Reply #4 on: October 12, 2013, 12:05:10 AM »

Have tried to tune it without the aid of the ATU but the SWR is right off the scale.

I have the antenna on a mag mount, which I now found out Diamond do not recommend!
What I have to do is get a good earth run from the thread on the SO-239 to an earth bolt somewhere on the vehicle before I start chopping.

The centre frequency I want is 7.122mhz. 0.077 to the left will give me 7.045 and 0.077 to the right of the centre frequency will give me 7.199 which these two points is the bandwidth I want to work within.

In the yeasu transceiver manual it also states to use the ATU on all TX although the antenna is a perfect impedance match for the radio.
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M6GOM
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« Reply #5 on: October 12, 2013, 03:00:22 AM »

Have tried to tune it without the aid of the ATU but the SWR is right off the scale.

I have the antenna on a mag mount, which I now found out Diamond do not recommend!
What I have to do is get a good earth run from the thread on the SO-239 to an earth bolt somewhere on the vehicle before I start chopping.


I wish you'd mentioned that first. I assumed you were just using the ATU to give you a bit more bandwidth. Forget everything I've said. Depending on the mismatch and the power you've been running you may have actually damaged the coil in the antenna forcing it to work with an ATU.

OK, assuming the antenna is OK then back to the problem at hand.

You basically have half an antenna. A magmount isn't going to provide enough capacitive coupling and neither is running a bit of wire from the thread to a bolt. Remember RF runs over the skin of the conductor so you're going to need to use something with a large surface area so flattened out braid from RG213 should be considered the minimum but again, it isn't really sorting the problem.

If you want it to work well you need to consider drilling a hole in the car and putting a permanent 3/8 mount in or at the very least using a lip mount such as a Diamond K400 with screws that bite through the paint to the metal below.

Please read this section of the K0BG.com website about mounts.
http://www.k0bg.com/antmount.html

Also have a read through other sections on the website especially in regards to bonding etc.

http://www.k0bg.com/

Until you've sorted out a better mounting and grounding solution all you'll be doing with anything you try to do to sort it out is chasing yourself round in circles.

You can "get away" with using a triple magmount on 20m and above. You can get away with using a single magmount on 10m. You can't get away with using a magmount at all on 40m without some problems even with 10W and definitely not on 80m.
« Last Edit: October 12, 2013, 03:03:15 AM by M6GOM » Logged
M3RLQ
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« Reply #6 on: October 12, 2013, 01:47:05 PM »

All understood. Have been down to my local radio ham shop and picked up a Diamond k400, a SO-239 connector and some good earth bond. This is going to be my next project so I'll you know how I get on.

Thanks for the links..
« Last Edit: October 12, 2013, 02:22:03 PM by M3RLQ » Logged
M6GOM
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« Reply #7 on: October 13, 2013, 08:36:23 AM »

Where are you planning on mounting the K400?

If it is on a hatchback or a boot lid you need to bridge the hinges with a bonding strap.

I did mine using flattened RG213 braid and crimp connectors (both crimped and soldered on) to make the straps and using star washers to cut through the paint.

Here's a picture to give you an idea...

http://i300.photobucket.com/albums/nn9/computershack/Amateur%20stuff/Icom%207000%20install/4Uu89.jpg

I did the same on the bonnet as well but because of the length I used wider braid...

http://i300.photobucket.com/albums/nn9/computershack/Amateur%20stuff/Icom%207000%20install/20120811_210414.jpg

When you're doing bonding to tell its working and when you've done enough do the following - you'll need an antenna analyser and most clubs have one they lend to club members. You do not need to tune the antenna to do this before you start.

1) With the antenna connected to the mount use the antenna analyser to find the resonant frequency where X=0 or as near as. There can be a few points near the resonant frequency where X drops but doesn't get to zero so don't take the first dip as being right. If you've got multiple points where X is low but never gets to 0 then use the one where the impedance (Z) gets to or near 50 Ohms just slightly higher.

2) Add a bonding strap.

3) Repeat 1. You should notice that without altering the antenna at all the frequency of X has lowered which shows that your bonding is working and having an effect. I noticed a drop of 100kHz when I put the first strap on the hatchback as per the picture and again about the same when I did the bonnet.

NB you may not notice a difference when for example you do the other side of the hatchback/boot lid.

Once you've added as many as you can so it makes no more difference adding more or that you've done as many as you're willing to at that point then you tune the antenna for lowest SWR at the frequency you want.
« Last Edit: October 13, 2013, 08:39:51 AM by M6GOM » Logged
M3RLQ
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« Reply #8 on: October 13, 2013, 10:56:30 AM »

The K400 is going on the hatch at the roof  top between the two hinges. Im using half inch copper braid for the earth to the hinge bolt. Coax to the pl-259 and so-239 have all been soldered and tested for any shorts. SO-239 is now bolted to the K400. Now waiting for dryer weather to attach mount to car, probebly the weekend now as work will get in the way.

When you say earth both hinges is that from hinge to hinge or from the top of the hinge to the bottom of the hinge?
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G7MRV
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« Reply #9 on: October 14, 2013, 08:12:46 AM »

GOMs advice is sound. The more you bond each of the vehicles panels, especially those that are horizontal, the better your RF ground will be, and consequently the lower your resonant point.

You need to bridge over the hinges, to join the panels together, as the hinges will be a very poor contact.

Im in the process of improving mine (I drive a fiesta) and have the doors to do still, and the exhaust system. The bonnet and boot are already bonded. I also have a permament SO239 through panel mount through the roof (I have no qualms at all about drilling!)

I also have just bought a 40m 'hamstick' type antenna, arrived this morning, as my commute is now mostly in darkness!. Bear in mind the size of these antennas. I found on my mounting spring these would not stay up at very great speed! and I dont like the idea of not using a spring due to the thinness of modern roof panels. To get around this, I will use a thin strong guy line when using the 40m antenna.

Bear in mind also that these antennas are not 100% efficient. I believe as a foundation you have a maximum of 10w, so the better you can make your ground system the more of that you will radiate. As well as improving your ground, have another ham help you on air to set up your tx audio for the punchiest signal you can, without distortion, this will help make the most of your limited power.
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M3RLQ
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« Reply #10 on: October 14, 2013, 12:22:37 PM »

I too have a Fiesta, 2008 model and looking at the hatch im thinking I'm going to have problems putting the mount where I first wanted it. I wanted to mount the bracket in the center top of the hatch door but the way in which the hatch opens and closes will prevent me to mount it there (bummer).

In the worst case I may have to mount it at the side of the hatch, right beside the Fiesta name badge. It's far from   ideal and not central. I can't even mount it roof level on the hatch as the back window comes right to the edge of the hatch leaving no metal to mount.

Another thing to mention is this setup is only for stationary work meaning the antenna would not be connected while on the move to prevent unnecessary drag and strain on the antenna and mount.

I really need to have a good look at my options when the weekend arrives, more time to tinker about.
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M6GOM
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« Reply #11 on: October 15, 2013, 11:39:58 AM »

The K400 is going on the hatch at the roof  top between the two hinges. Im using half inch copper braid for the earth to the hinge bolt. Coax to the pl-259 and so-239 have all been soldered and tested for any shorts. SO-239 is now bolted to the K400. Now waiting for dryer weather to attach mount to car, probebly the weekend now as work will get in the way.

When you say earth both hinges is that from hinge to hinge or from the top of the hinge to the bottom of the hinge?

If you look at the picture I posted, it is from the tailgate side to the body side and you do that on each of them.

Because the bottom half of the tailgate below the high level brake light on a MK4 Mondeo is all plastic and I've not been happy with the efficiency of my tailgate as a ground (previous car I had my Little Tarheel mounted through the middle of the roof) since I discovered that I've drilled a hole into where the roof starts to come down where the tailgate is and added another braid strap between there and the mount. Unfortunately my antenna analyser was out on loan so I couldn't check to see if it made a difference but I'll be doing that again soon.

You may want to make sure that where you're planning to mount it next to the Fiesta badge is actually metal if its the newer shape one.

Another consideration is common mode. You're going to have some on the coax so you may want to look at putting some 31 mix ferrites on the coax before it enters the vehicle if you notice any anomalies or someone tells you your audio sounds distorted..

By the way those K400 mounts are damned strong. Someone wrove my screwdriver antenna off my car. They had to use both hands and when I got it back it was still on the mount and the mount hadn't moved. The metal on the tailgate gave way. So you've no worries about driving down the road at 80MPH into a force 9 head wind and having the thing swivel or tilt.

Here's a picture of my Little Tarheel II on my car. Its got a 6ft whip on and smacks a tree twice a day. Never moved a bit.

http://i300.photobucket.com/albums/nn9/computershack/Amateur%20stuff/Icom%207000%20install/20120811_202650.jpg

You can just about make out my VHF antenna in the middle of the roof. The Tarheel would've ended up there had the roof not been so thin. Unfortunately what they use for sound deadening on the underside of the roof prevents putting some sheet steel underneath to strengthen it. Modern cars are getting harder and harder to mount HF antennas on even if you are willing to drill holes like I am.
« Last Edit: October 15, 2013, 11:52:45 AM by M6GOM » Logged
M3RLQ
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Posts: 15




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« Reply #12 on: October 18, 2013, 02:17:04 PM »

Good news,

The antenna is tunned at 7.122mhz  swr is 1:1 @ center frequency without ATU in line. Obviously the swr creeps up either side of center frequency as the antenna has a narrow bandwidth but the ATU tunes it 1:0 with ease now. The mount is on the hatch right beside the Fiesta badge. Getting the braid clamped under the mount was a pain as the bracket is designed for flat sheet metal.

All works well.

I would post a pic if I knew how!
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M6GOM
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« Reply #13 on: October 18, 2013, 02:39:37 PM »

Thanks for keeping us updated.

As for the pic, easiest way is to upload it to Flikr/Photobucket/Imgur or whatever and post a link.
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M3RLQ
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« Reply #14 on: October 18, 2013, 03:30:57 PM »

Excellent,

Thank to all.

M3RLQ
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