Open-wire-fed long wires - Range of probable Z

[NQ8Z]

My portable 40m QRP antenna is a symmetric long wire fed with an open wire feedline. Typically hung at 20-30 ft. The antenna is about 100 ft long, the feed line isn't very long, maybe 40 ft.

In a very general way, what range of impedances am I likely to encounter at the transmitting end?

For example, at the transmitting end, are the impedances almost certainly going to be greater than 50 ohms and reactive (L or C)? But of magnitude less than 500 ohms?

[K3WRV]

There used to be a chart in the ARRL Handbook of impedence vs height. I suppose you could put an antenna analyzer on it or use a modeling program, but I once worried about this and was persuaded not to. For QRP from the field, I mostly feed the ant with 20 Ft or so of coax, and just use an L network turner to try to make it work, and mostly it has. I mostly use a 40/20 twqinlead deipole at whatever Height I can get, figuring I've got 40, 20 es 15 covered. But it also worked on 10 (NOMEX ON - I KNOW that wont work) But worked a bunch of Euro DX from the middle of SD! With a barefoot 509 Argo.

When doing QRP Portable, don't worry about it. Just try your best, and you will make contacts. A lot of QRP guys use RG 174 as feedline when portable, and look at the specs fer that? (BIG LOSSES!) (but it sometimes works, FOR PORTABLE!)

Sayeth he who hath fed balanced line to unmoidfied ARC 5's.

{EDIT] Sorry for not answering your question. I guess my respponse is "why worry about that - it will be different when ever you put it up - QRP TTF is a bit quirky. But a LOT of fun.

[K8JD]

Why don't you just make a resonant dipole tuned and pruned to best swr at the 20-30 ft height. You can feed it with 40 ft of RG58 without having to take extra tuners and such if you are operating portable. Just coil up the wire and coax, pack it with rig and battery and GO!.
You know that will work without a lot of hassle when getting set up and have more time to make contacts at your portable site if that is what you are considering.
One summer I won 3rd place in a SKCC sprint contest with a TS520 and a 40M dipole I made on site and hung in trees at a campground site. Took about half an hour to set it up.

[WA5KRP]

Looking at Fig. 3, Page 2-4 of the ARRL Antenna Handbook (21st Edition), shows an impedance range of 200Ω + j1000Ω at 7MHz to 1000Ω + j1500Ω at 8MHz using 100' of 0.1" diameter (#10AWG) wire. This is an isotropic antenna in free space made with relatively large wire. Your antenna most likely is smaller in diameter, such as #16AWG or #20AWG, and will have higher impedance.

What your tramsmitter sees at the load with 40' of 450Ω twinlead (if that's what you have), according to TLW and the numbers above:


7.00MHz: 34.0Ω - j7.2Ω with an SWR of 13.2:1

8.00MHz: 76.6Ω + j221.5Ω with an SWR of 7.3:1


Of course you will operate well below 8MHz, but you can see the SWR range between 7 and 8 MHz is perfectly acceptable for window line with 97%+ of your power going to the load. How well your QRP rig can load 450Ω line is a question mark, but a very simple L-network will match it for you.


Good DX to you!



WA5KRP
Texas

[W5DXP]

My portable 40m QRP antenna is a symmetric long wire fed with an open wire feedline. In a very general way, what range of impedances am I likely to encounter at the transmitting end?

Assuming that your "symmetric long wire" is a dipole that is at least 1/2WL at its lowest frequency of operation, you can get any impedance between 25 ohms and about 8000 ohms with reactance. If you can read a Smith Chart, you can get a rough idea from this chart of mine representing a 130 foot dipole.

http://www.w5dxp.com/smith.htm

[NQ8Z]

W5DXP's information is very useful and scary. It's a 320:1 matching range, well beyond what's easy to see on a Smith chart.

I've been trying to find values for a "match everything" pi network. Nothing realistic comes near doing this, even at QRP levels. A switchable 4:1 Z transformer leaves the matching range at 80:1. Relative to 50 ohms and with the transformer in, 8000 ohms is 40 at the very edge of the chart. And, don't forget to switch it out in the 25 ohm case ( 0.13 )!

Yet my EmTech ZM-1 matcher has no problems matching a 200 ft symmetric long wire at 60 ft fed with open wire. It matches perfectly 80m on up, no problem.

I'm wondering if the pi circuit is suitable.

[KB4QAA]

disregard

Bill

[W5DXP]

W5DXP's information is very useful and scary. It's a 320:1 matching range, well beyond what's easy to see on a Smith chart.

For 1/2WL dipoles on the lowest frequency of operation, you can lower that matching range to 4:1 by varying the length of the ladder-line. The range of impedances to be matched drops to 25-100 resistive ohms. For me, throwing a few knife switches is more than worth the elimination of matching problems.

http://www.w5dxp.com/notuner.htm

Yet my EmTech ZM-1 matcher has no problems matching a 200 ft symmetric long wire at 60 ft fed with open wire. It matches perfectly 80m on up, no problem.

You may have (accidentally) stumbled upon a good combination of dipole length and ladder-line length. For instance, 100 feet of 450 ohm ladder-line is a good length combined with a 130 foot dipole. You could analyze your antenna using the free demo version of EZNEC available from www.eznec.com

With a 200 foot dipole and 67 feet of 450 ohm ladder-line, EZNEC says the impedance at the tuner will be 33 ohms resistive. I'll bet your ladder-line length is in the ballpark of being that optimum 67 foot length.

If you had chosen 125 feet for your ladder-line, the impedance at the tuner would be in excess of 6000 ohms.

[NQ8Z]

W5DXP called the length of my feed line correctly. Certainly 33 ohms is within the matching range of any reasonable matching circuit.

Adjusting feedline lengths isn't feasible in my field application, thus a tuner seems appropriate.

My calculations displayed on a Smith chart suggest that a pi network can match a wide range at 3.5 MHz only for large (~2000 pf) values of C1, C2. These values of C would require switching in fixed capacitances as well as L taps. And, with a 320:1 impedance range, there's a reasonable argument for a switchable output transformer as well. At some point, the possibility of finding a match becomes very small.

Any ideas for a practical pi match for balanced feed 80 - 15 m? I've calculated the matching range for the highly recommended Collins 180S-1 and it doesn't seem to do so well on 80 m, either.