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Author Topic: 160m coil design  (Read 2374 times)
KC8QVO
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« on: December 31, 2009, 09:59:54 AM »

I am wanting to put a coil on top of my screwdriver antenna that will let me get on 160m. However, I am having some difficulty in calculating the values needed.

I posted a thread on qrz.com but no one has replied. Based on the lack of replies, yet high hit count, I don't think anyone over there knows how to tackle the project. Just so I am not re-writing everything here is a link: http://forums.qrz.com/showthread.php?t=230997

In a nutshell, I need to know how much inductance I need to get my antenna to resonate on 160m and then design a large coil to that inductance. Are there any easy formulas out there? As is in the other thread I tried to use a page from ON4UN's Low Band DX'ing but had problems with the calculations. I just want to ballpark it. I can fine tune it later.

Steve, KC8QVO
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K0BG
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« Reply #1 on: December 31, 2009, 10:12:45 AM »

If you had an ARRL Handbook or Antenna Book, you'd have your answer.

The inductance required to resonant an 80 meter antenna on 160 meters varies with the overall length. However, it'll be about 4.5 times that needed for 80 meters. If the antenna is 9 feet long (really short for 160), the additional inductance will be about 525 uH.

You'll need to design the coil to have an LD ratio (length to diameter) of about 4:1 for best Q. A diameter of about 3.5 inches is about right, so the length will be about 15 inches.

The wire size, and TPI will determine the inductance. That you will have to calculate yourself. As I said, start with the ARRL Handbook.
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K0BG
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« Reply #2 on: December 31, 2009, 10:29:33 AM »

After reading your post on QRZ...

I wouldn't use aluminum wire if for no other reason than Q.

You would want to add enough inductance to resonant the antenna on the bottom of 160 with the screwdriver near the bottom of its travel. Even then, you probably won't have enough adjustment to cover all of 160.

The 200A is a decent screwdriver, but I'd be a little afraid of sticking that much weight atop it in any case. What's more, if you really want 160 meter efficiency to exceed about 1%, you're going to need a rather large cap hat. This fact also negates the hamstick whip idea.

Personally, I wouldn't attempt this project. If you just have to, why don't you call Robert at Tarheel, and see if he'll recoil your 200A with a 160 meter coil? If you do this, remember, your upper band efficiency will suffer over what it is now, by as much as 6 dB depending on how well you have the antenna mounted in the first place.

There is no free lunch, especially when it comes to 160 meter mobile antennas. So, think about it a while, before you jump in with both feet.
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K5END
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« Reply #3 on: December 31, 2009, 11:52:59 AM »

...if you really want 160 meter efficiency to exceed about 1%...

Educated guess, but 1% efficiency for 160 meters on a mobile antenna is pretty close to as good as it gets.

Maybe he could build a fiberglass "18 wheeler" semi-trailer for a low horizontal dipole and pull it with a tractor.

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KC8QVO
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« Reply #4 on: December 31, 2009, 12:01:23 PM »

The aluminum wire is what I have. I got a whole spool of it for free. So since I have it I am going to use it. Is there any neumerical ratio beteen coil Q of aluminum vs. other materials in the same design? Are we talking 50%, 80%, 2% of a "good material"?

I checked the antenna book and it says 345uH for base loading and 700uH for center loading on 160m with an 8 foot whip. Your 525 (I think that was it) is right in the middle. So that seems about right. I still haven't run any meaningful calculations though. I think I would be better off building and tuning with a bit of an over estimate.

I have 4' of bottom mast that is 3" in diameter and about 80" of whip on top (a short and long buddipole arm plus a 4' hamstick whip). I have a 30" cap hat mounted between the long and short buddipole arms. So the effective "antenna" length is a bit longer than 10'. I would guess the 525uH would be about right.

You mention that the best coil Q is with a 4:1 radio of length:diameter. One of my friends has always said 1:1 ("square") and the antenna book says no less than 1/2:1 "for best Q". So what is the right method?

Steve, KC8QVO
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K0BG
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« Reply #5 on: December 31, 2009, 12:51:16 PM »

First, Larry is correct. If you do everything right down to the numbers (15 feet overall, Q at least 300, a very large cap hat at the top of an 8 foot whip, and with lots of metal mass under the antenna), you might get close to 3%. In any case, you won't get 1% using aluminum wire in the coil.

The optimal LD starts increasing as the total inductance gets over about 50 to 70 depending on the original LD, where in the antenna the coil is located, etc. By the time the coil is around 700 uH, the ratio is closer to 5:1 with copper. I didn't run numbers for aluminum (it would be slightly different), but the Q would be much less.

As I said before, your best bet is to have the coil replaced, or buy a 160-10 screwdriver. Going about this in the fashion and form you're attempting, just isn't practical electrically or mechanically.
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KC8QVO
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« Reply #6 on: December 31, 2009, 05:47:59 PM »

Alan -

I appreciate your information and concerns. Though, Ham radio is a great hobby for experimenters and I am experimenting HIHI. If the coil ends up being too much for the screwdriver to handle alone I will try guying it. More experiments to come I am sure.

Though, I will say that I have had 11' of whip on the screwdriver going down the road. I had the whip bent over the front of the truck so it wasn't all sticking up, but the screwdriver did fine none-the-less. The only issue was the hollowed out 3/8"x24 studs on the buddipole arms. I had to replace one after it broke (stainless steel, no breaking that sucker - the arm will go before the bolt does).

Steve, KC8QVO
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WX7G
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« Reply #7 on: December 31, 2009, 07:55:18 PM »

Is this for mobile operation or is this a ground mounted screwdriver?

The reason I ask is that for ground mounting DX Engineering has the components to put a screwdriver on top band?

If for mobile tell me the whip length and I'll calculate the coil inductance required. Think about using a 108" whip.
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KC8QVO
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« Reply #8 on: December 31, 2009, 10:43:30 PM »

Well.. the antenna is mounted to my truck. Whether or not it goes down the road with this set up is to be determined HIHI.

As for the whip - I have all kinds of parts to play around with. I could go up to 16' if I wanted. Then again, mobiling at that height would be problematic HI. I have done that though - I put the two buddipole arms I have on the bottom, a 10m hamstick base, and a 9.5' telescoping mast. I set it up like that in the driveway for some better efficiency on 40m one night. It worked.

Steve, KC8QVO
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WX7G
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« Reply #9 on: January 05, 2010, 07:52:53 PM »

One off the shelf way to go is to use a Hustler 72" mast extension, a Hustler RM-80 resonator, and a DX Engineering top hat. The top hat with six 46" spokes will work. Tune it near the top of the 160 meter band with the screwdriver at minimum inductance. Then adjust the screwdriver inductance resonance.

The minimum VSWR might be 2:1. If this is too high a base shunt inductor can be used to increase the input impedance to 50 ohms.

This is what I use for the top band antenna at home.
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KC8QVO
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« Reply #10 on: January 09, 2010, 05:56:04 PM »

If I can make a coil that will work why would I buy the parts to do it?

Steve, KC8QVO
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K0BG
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« Reply #11 on: January 10, 2010, 05:46:28 AM »

That's the challenge; finding the parts. Few ops realize the stress applied to a mobile antenna, especially time related ones. Poor design always leads to failure; it's just when that is in question.

No one make, sells, markets, etc. any parts which could be considered as loading coil parts. Now that TBC is out of business, you can't even buy fixed coils.

You can use PVC, but the end connections become a hassle. Further, unless you own a lathe, about any fitting you use will be a kludge.

All of this is why I suggested you just have the antenna coil replaced with a 160 meter one. While experimenting is part of amateur radio, one also has to be practical.
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WX7G
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« Reply #12 on: January 10, 2010, 01:48:14 PM »

Aluminum wire no good for a coil? Is this another myth to be debunked? Compared to copper the coil Q will be reduced by a factor of 1/3.

For example a Q of 300 copper coil will turn into a Q of 200 aluminum coil, give or take some.

Work the numbers remembering to use a ground loss resistance of 5 to 20 ohms for mobile and see how much signal strength an aluminum coil will cost you.

Example:
Given a 200 uH copper coil having a Q of 300 and replacing it with a 200 uH aluminum coil having a Q of 200. Let's work it for an 8' top loaded vertical at 1.8 MHz. Rr = 0.4, Rgnd = 10 ohms.

Efficiency = 2.2% copper coil
Efficiency = 1.8% copper coil
Difference = 0.9 dB

You decide if 0.9 dB is worth going to copper.
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WX7G
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« Reply #13 on: January 10, 2010, 02:04:49 PM »

KC8QVO, if you post dimensions and where you plan to place the coil I will model your antenna, tell you the inductance, and how to build a suitable inductor. A description of the top hat is needed.

The Q ratio between a copper coil and an aluminum coil is 3:2.
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WX7G
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« Reply #14 on: January 10, 2010, 03:17:09 PM »

Correction:
Efficiency = 2.2% copper coil
Efficiency = 1.8% aluminum coil
Difference = 0.9 dB

Personally I prefer to use close wound #14 THHN wire. Very easy to wind and duplicate and the Q can be 300.
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