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Author Topic: Can you recommend a backpackable 40M QRP homebrew design for my BoB?  (Read 1616 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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