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Author Topic: Can you recommend a backpackable 40M QRP homebrew design for my BoB?  (Read 2113 times)

SlowBro

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Hello! I've looked at ham radios since I was a kid but this week bought my first rig, a Pixie 2 QRP kit off Fleabay. (I know, I know. Not the best, but money is tight.) I'd like to grab and pack it in my BoB or bike trailer if there is a crisis, and I'll be keeping it in an EMP-resistant box until needed. Would be battery and/or solar-powered.

I've looked at various antenna designs and can't make a decision. Can you recommend an antenna type that meets the requirements below?

I'm "leaning" (pun intended) toward going with a tree-supported sloping Windom. Just slingshot a rock with line up a tree and pull up my wire. This should let me put the short end near the ground for less coax needed, and I already have a TV balun. (75 ohm - 300 ohm, that's 4:1 right? New at this.) They're supposed to be good for DX and when I want to change direction I just walk around the tree :-)

  • Backpackable e.g. less than a few pounds/kg and rolls up into a smallish package
  • Quick setup/tear down
  • No poles, will use trees
  • Less than USD $20 please. I already have some telephone wire, some TV coax, some Ethernet wire, a TV balun, some thin stainless fishing line. Oh and I bought a BNC balun for 75 cents that is typically used for CCTV to transmit over CAT5. (I don't know its ratio but I guessed 1:1?) I figure worst case I can hang the transmitter right on the dipole, but on the other hand that'd require me to place the dipole close to the ground to reach the transmitter.
  • Feed line any type but partial to TV coax for cost savings. The transmitter output is 50 ohms with a BNC connector so I bought a BNC-to-F adapter to let me use 75 ohm TV coax. (75 cents.) From what I've been reading, the feed line loss going from 50 to 75 at 7MHz is acceptable and keeps within 1.5:1 SWR.
  • No tuner required, I'm sure I can ask around to find a SWR meter at the local clubs to tune it up once built.
  • Single band is acceptable. If absolutely necessary I can switch the crystal down to a shorter wavelength, also requires replacing the inductor on the Pi filter. I've read these Pixies can go down to 6M. But I want acceptable performance year-round so I'm trying to steer clear of higher frequencies and sunspot concerns.
  • Would like to be able to receive and perhaps transmit a few hundred miles/km but will accept loss of distance as trade-off for price and weight/size
  • Likely be doing more DX than local, getting an international perspective during a crisis could be useful. But I do also want to know of regional conditions such as nuke plants after an EMP, so maybe I do want to stick to local? I'd like your opinion on this as well. If local, could an NVIS be a better option?
  • I think I would be doing LOTS of listening and not much talking so biasing the design toward receive would be better.

Am I asking for a unicorn or is there a design with decent compromises for my situation?
« Last Edit: August 11, 2017, 02:19:10 PM by SlowBro »

SlowBro

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #1 on: August 11, 2017, 02:47:23 PM »
The G7FEK looks possibly useful. String the two ends between two trees 24 feet off the ground; I can do that. Good for DX. Needs 24 feet of ladder line or twin feed, which goes over my budget, unless I can find some wire at a thrift store or garage sale. No tuner is required with careful construction.
« Last Edit: August 11, 2017, 08:20:15 PM by SlowBro »

SlowBro

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #2 on: August 11, 2017, 05:45:46 PM »
Also looking at on-ground or slightly above-ground (Beverage-style) antennas. Maybe one that can be both Beverage and NVIS depending on the height and configuration? There was an article I once found (and can't find it now) antenna I saw that was I think full wave that sat directly on the ground. I'm thinking some fine magnet wire perhaps could make that work for a backpack...

Edit: Ahh, here's that article. Also describes laying a Windom in the grass.

I picked up some ferrite bars just now to DIY the balun in case the TV balun doesn't do the trick. (I have a strong feeling it won't.) This article describes how to take a short length of speaker wire and wrap a balun.

So with those ferrite bars and speaker wire I should be able to use my length of TV coax, the BNC-to-F adapter, and string a Windom one of four ways: Up high if I can find two tall trees, sloper if not, low for NVIS, and on the ground ala grasswire. I like it. I'll try to update how it works, and please, if anyone has better ideas let me know. It'll be a month or so before I can test, as the parts are currently on the slow boat from eBay heaven.
« Last Edit: August 11, 2017, 08:13:57 PM by SlowBro »

gil

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #3 on: August 12, 2017, 04:34:53 AM »
Hello,

I would recommend an adjustable end-fed tuner with a half-wave wire. Look at the ILERTENNA from EA3GCY, or the MEF-1 single-band tuner. There are others, from qrpkits.com or qrpguys.com...

A Windom will often have common-mode-current issues... You need a good BALUN and choke, more money...

Gil.

SlowBro

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #4 on: August 12, 2017, 07:31:10 AM »
Hmm a tuner... The MEF-1 looks good. I found the schematic and I may try to just make it, since it looks so simple. Put an order in for the parts since they were cheap enough.

As to the Windom, this article shows making one with a homebrew balun and no choke, claims "this antenna is compromise but in practice works very well" and "On 40 m band the antenna is about -6 Db (1 S) lower of a full size dipole but in practice it has the same behavior on locals and DX stations."

But if I test it and find it still demands a choke, I already have a bag of snap on ferrites so I will try that. There's also the method of wrapping coax around PVC and even doing both at the same time.

So now I can try a few different configurations and see what works best, and I didn't blow budget. Thanks!
« Last Edit: August 12, 2017, 07:54:38 AM by SlowBro »

gil

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #5 on: August 16, 2017, 06:42:31 AM »
The Windom does work well, but I found that different lengths of coax changed the SWR wildly...

Gil

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SlowBro

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #6 on: August 16, 2017, 07:53:21 AM »
On your advice I picked up the parts to make a tuner with SWR LED indicator. Simple and cheap circuit. For the antenna wire I will solder in spade connectors to alter the config between Windom, end fed, and dipole. Thus I won't have to pick only one configuration. I'll carefully match coax in the New Carolina Windom style having its necessary 10' of coax vertical, and a choke of ferrite snap ons and coiled coax at the end. That choke should only benefit the end fed and dipole configurations so it should make a great three-in-one portable antenna.

The radiator wire and center balun will be loosely taped to a very long fishing line. That line will do all of the load carrying and the wire can thus be thin (lightweight) and use spade connectors that would otherwise disconnect when strung between trees. No stress or load carrying on the radiating components. A rock on both ends, slingshot the ends into trees, pull both sides down, and tie it off.

Testing in a month or so and I'll try to remember to update this thread with my results.

By the way forum notifications don't seem to work. I don't get emails when someone replies.

gil

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #7 on: August 16, 2017, 08:25:37 AM »
Don't forget to post about it, with photos ;-)

Gil

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cockpitbob

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #8 on: August 24, 2017, 11:51:10 AM »
For light weight portable, I'm 100% End Fed Half Wave.  You can rig it as a vertical, horizontal or inverted-V.  The feed line is short, or none at all, since you connect to one end instead of the middle.  And unlike dipoles and Windoms, with an EFHW antenna you only have to get one line over a tree branch.  It's the smallest, lightest and most adaptable antenna you can carry, and they work great.

The adjustable tuners like the qrpkits.com SOTA tuner are good, but I like the ones with a simple wide-band transformer similar to the LNR Precision and myantennas.com designs.  A 63' piece of wire and 6' counterpoise get you 40/20/15 since they are harmonically related.  If you shorten the wire you have 30.
Here's a nice short write-up on a home-brew wide band transformer:  http://vk3il.net/projects/efhw-matching-unit/

To get a line over a branch I either use a collapsible fishing rod or slingshot with fishing reel hose-clamped to the wrist support.  If the antenna is staying up for more than a few hours I use mason's line (thin but 100lb+ test).
« Last Edit: August 24, 2017, 11:52:53 AM by cockpitbob »

SlowBro

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #9 on: August 24, 2017, 12:09:07 PM »
Right on cockpitbob. That looks like a good article. I'll review it before I settle on the final tuner design.

I had discovered the same since opening this thread. I had not given an end fed much thought until Gil recommended it and now I believe that's what I will carry in my pack. As you said, I don't need any coax at all and in fact I will just connect the antenna directly to the PCB.

The Pixie's BNC connector won't be installed and instead, two wires to another PCB which holds the tuner (likely the MEF-1) and SWR indicator circuit. A hole in one side to lock in the antenna wire strung up and out and another hole for the counterpoise. Will keep short lengths of wires permanently connected to the board and bullet connectors to add on the rest of the radial and 0.05 wavelength counterpoise. Plus the aforementioned fishing line to hold it up and take the strain. (Mason's line, you say. Interesting.)

I like the way they run the wire on the MEF-1. Takes all the strain off with no added components. By the way I found this page which describes a variety of ways to string an EFHW.

Other ways to get the line over the branch: A plastic water bottle, line tied around the neck. I like this once since there's nothing extra to carry; my pack has water bottles. Another way: A nylon sock with rocks in it. Tie the line around the neck of the sock. I may be carrying a nylon sock anyway, but the thought of it snagging and losing maybe my only other sock, I don't like that.

Also, I initially said "no tuner or SWR meter" because I saw them as heavy, expensive black boxes. Didn't know they can be made very cheaply with very few components 8)
« Last Edit: August 24, 2017, 12:17:09 PM by SlowBro »

swxx

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #10 on: August 24, 2017, 10:49:57 PM »
SlowBro, I could be wrong, but I wonder if just connecting an end fed with 20m long (half wave) to that Pixie directly is going to do any damage, and whether the 300mW to 1W (check it's output with a proper QRP power meter, it may be less than expected) will go into such an antenna. Failing that a 10m wire as high as you can get it and a 10m counter poise is in effect a dipole and will be around 50 ohms match. Also I'm wondering why build a multi band difficult to make antenna like a G7FEK if you can just use a simple dipole cut for 40m and fed with the very thin tiny coax?

If you have a squid pole I'd run a wire up it, with a few turns to make it fit and be resonant, and feed that with thin coax. Better, get a short piece of plastic pipe that will fit over the squid pole but get stuck say half way up due to its diameter, and on that plastic you wind many turns of wire and then the rest of the wire loosely wound up to the top of the pole. Experiment with how many turns you  need. On that plastic at the beginning of the windings, you connect your coax inner to the wire, and outer, to a radial, which can be 10m long or so for 40m, but it's not critical, and slope that down. If you can add more than one radial, as many as 3, even better for the radiation pattern off the vertical part. This is then an elevated vertical or ground plane antenna. Simple and works well for longer distances! Down side during TWAWKI the QRM level locally may be higher on a vertical. A resonant half wave dipole with coax will attenuate noise that is not naturally resonant at that band (40m). That does not thus eliminate marijuana grow lamps which it seems are all the rage in the current economic climate, but, once TEOTWAWKI hits, those will be off, the grass smoked, and the bands quiet. Aside perhaps from long range radars.

Did you mean 6m or 6MHz in your post, as to getting Pixie to work elsewhere? I think you meant down to 6MHz, that would make sense. I don't think it can be made to work easily or efficiently on as high as 50MHz?

swxx

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #11 on: August 25, 2017, 09:12:29 AM »
Slowbro this may also be interesting for the Pixie if connecting EFHW wire 20m long is not a good idea, then you can use this:
https://www.youtube.com/watch?v=yRedmR9urE4

SlowBro

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #12 on: August 26, 2017, 02:50:36 PM »
SlowBro, I could be wrong, but I wonder if just connecting an end fed with 20m long (half wave) to that Pixie directly is going to do any damage, and whether the 300mW to 1W (check it's output with a proper QRP power meter, it may be less than expected) will go into such an antenna.

I'm not going in directly, the impedance of an end fed is very high. Although it's been done before. No the antenna goes to the PCB and from there into a tuner then into the Pixie.

I'm wondering why build a multi band difficult to make antenna like a G7FEK if you can just use a simple dipole cut for 40m and fed with the very thin tiny coax?

It's the weight and expense of even thin coax. I like the end-fed, going to go with that first and see how it performs.

I think you meant down to 6MHz, that would make sense. I don't think it can be made to work easily or efficiently on as high as 50MHz?

I said 6M but I should have said 6m as in, six meters. I learned since creating this post that meters should be lowercase 'm'.

SlowBro

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Re: Can you recommend a backpackable 40M QRP homebrew design for my BoB?
« Reply #13 on: August 26, 2017, 02:53:01 PM »
Slowbro this may also be interesting for the Pixie if connecting EFHW wire 20m long is not a good idea, then you can use this:
https://www.youtube.com/watch?v=yRedmR9urE4

Yep that's the aforementioned MEF-1. I found the circuit diagram and bought the one-twosies parts I didn't have so that I can rebuild said circuit on my PCB.

I just got my Pixie in the mail and if I get my act together I'll post pictures and a status update.