Radio Preppers

General Category => New To Radio => Topic started by: SlowBro on August 12, 2017, 01:27:38 PM

Title: Do I need an SWR meter for a backpack rig?
Post by: SlowBro on August 12, 2017, 01:27:38 PM
Just bought a 40M QRP Pixie 2 kit off Fleabay. (Yeah yeah I know, not the best, but it'll do for now.) It transmits at under a watt. I'm aiming to keep it and either a Windom or maybe an end fed half wave in my BoB or bike trailer.

Since I may be placing these antennas in different configurations and heights depending where I can find a place to set up, I can imagine the tuner would need to be tweaked often for the best signal. Is an SWR meter strictly required in this situation or can tuning be guesstimated based on prior experience? I would think it'd be required if I used higher power, but I can't see that low wattage doing any damage to the rig if the antenna is not in an optimal location. I would just guess the tuner based on what worked in the past, for that particular antenna height.
Title: Re: Do I need an SWR meter for a backpack rig?
Post by: SlowBro on August 12, 2017, 08:15:54 PM
Say for example I'm at home and string my antenna up a half wavelength in the trees and find that I need to put the tuner dial 3/4 of the way. (I'm building a simple MEF-1 tuner. Schematic (https://drive.google.com/file/d/0B1pQlp8j8NqVdldmbnhNQTNOR28/view).) If I then go out into the woods and put the antenna at about the same height, can I reasonably expect that placing the tuner dial in about the same spot will give about the same SWR? (Knowing that perfection is not required.)
Title: Re: Do I need an SWR meter for a backpack rig?
Post by: SlowBro on August 13, 2017, 05:54:14 AM
Probably a moot question. I just found a circuit (http://qrpkits.com/files/EZ_SWR20170527.pdf) that's super easy to build using stuff I already have on hand and that would vanish into the pack, it's so small and light. I still would like to know the answer, but more for academic reasons.
Title: Re: Do I need an SWR meter for a backpack rig?
Post by: Jon_Garfio on August 13, 2017, 07:58:08 AM
For this qrp activities with a rig without swr meter, I use a endfed antena tunner that tunned throught a variable capacitor or coil, no need swl meter because the perfect tunned is when the maximum noise recepcion.

You can get it the mountain tunner of Sotabeams or the Ilertenna from a spanish dealer.

The ilertenna has a led to sintonize it (minimun bright is the perfect swr tunned).


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Title: Re: Do I need an SWR meter for a backpack rig?
Post by: Jon_Garfio on August 13, 2017, 07:58:48 AM
(https://uploads.tapatalk-cdn.com/20170813/7dac2c5981b674ea9e434a2613f7c79e.jpg)


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Title: Re: Do I need an SWR meter for a backpack rig?
Post by: SlowBro on August 13, 2017, 09:29:28 AM
Yes that looks like the circuit I mentioned in the third post. The LED is actually the SWR "meter" and its brightness changes strength as the reflected power changes. Quite a simple and clever circuit, and should be simple to build and cost little and weigh next to nothing and take up little room in my pack. A win-win.
Title: Re: Do I need an SWR meter for a backpack rig?
Post by: vwflyer on August 13, 2017, 12:52:17 PM
I own both the Sotabeams Mountan tuner and the SOTA Tuner. The second one has the LED indicator. When I hook these to my YouKits radio which has it's own SWR meter I find that the LED does a great job of giving me low SWR on the YouKits meter. However, with the Sotabeams Mountain tuner I found that simply tuning for maximum noise is not accurate enough to assure me low SWR on the YouKits rig. I have to use the rig's SWR meter to bring it down. So I wouldn't recommend a tuner without some kind of SWR indication if your rig doesn't have a meter.
Title: Re: Do I need an SWR meter for a backpack rig?
Post by: SlowBro on August 13, 2017, 06:53:27 PM
Ok but what about in the situation I describe? At home, I tune it. In the field, I put the antenna at about the same height. Would it be sufficient to say the tuner should be placed in roughly the same spot for both locations, or can they really vary that much?
Title: Re: Do I need an SWR meter for a backpack rig?
Post by: vwflyer on August 13, 2017, 10:57:33 PM
From one location to the next I don't have to move the tuning knob very far at all if I don't change my operating frequency. However, those tuners can have a pretty sharp Q. Often times the difference between low SWR and high is just a bump of the knob. If most everything about the two installations are the same (hight of antenna, kind of support, nearby objects, type of ground under it, etc.) it should be really close. But there are so many variables that can affect SWR and you will never be able to exactly duplicate your home setup. The hight of the tuner itself above the ground seems to make a lot of difference. Sometimes I'll stake the tuner down to the ground and sometimes I'll bungie it to a table or a branch. It just depends on my operating position's accommodations. When you set up in a randomly different location every time, you find different arrangements that seem to work best. Without a meter you won't know if it's close enough or not.
Title: Re: Do I need an SWR meter for a backpack rig?
Post by: SlowBro on August 17, 2017, 12:16:46 PM
Thanks all! I'll try to remember to report my success or failure later.