Topband: Inverted L with elevated radials for topband

Thu Mar 5 15:13:56 EST 2020

Hi Gabriel,

There is too many variables in your antenna setup to determine the expected impedance.

I have built and measured several inverted L's with/without elevated radials and/or sloping top wires.
So I will refrain from putting out estimations, other than guesses from what I learned during the last 30yrs.

What I do know:

Fact: an inverted L with 1/8 vertical and 1/8  straight  horizontal offers a radiation resistance  Rs=18 Ohm. No more.

Fact:  Antenna impedance (Z)= Rs+Rg.   Rground refers to energy not radiated but lost in the radials to ground. 

Your Antenna Analyzer/VNA will offer at resonance (find the frequency at which Xc=0) is Z.  You now have two unknown variables(Rs and Rg) and one measured value Z 

Fact: Z, SWR_usable_Bandwidth, Rground, Rs and any device to match it all to 50 Ohm are variables which can be a challenge. But don't sweat it.

Fact: making the vertical part higher v.s. sloping wire=> get the sloping wire up. That is much more effective. 

My experience:

- A straight horizontal L-part is just to about impossible; at 1/8 length any wire will droop in the middle and lower Rs substantially. Spent two days measuring just that when I had two 23m HD tubular towers.
-  A sloping single top wire quickly lowers the Rs. In your setup I would expect it to be between 11 and 13 Ohm

- knowing that guestimated value; when your analyzer shows Z=40 Ohm SWR=great at resonance (Xc=0) one can calculate the ground losses Rg=40-12=28Ohm. That will be with 4 radials on ground probably. A nice SWR but effectivity is  12/40= 30%  the other 28/40-70% of the power goes into ground.

- Good SWR does not mean the antenna is 'really good'. But don't seat it, sometimes you have limited options. And you have to start somewhere.

- Elevated radials: been there. 4 resonant elevated radials is possible, but difficult to measure correctly such that all four behave similar. We had  them at 4m height. Higher is better, more is better.

- If you lengthen the L-part so the total becomes 50 to 54m and tune out that extra length using a series Cap at the feedpoint, you can raise the Z. We did it with our 4 elevated radials and got a Z=34 Ohms which was nice to use a W2FMI 32:50 Ohm transformer.

- Radials; put down about 30-50pcs with 25 to max 30m length.  Want to lower Rground more effectively? Go to 120 radials at longer lengths, OR
-  Additionally put lengths of chicken wire directly under the vertical. Do not try to make length in a start config like radials; instead just closely cover straight lengths, make a square. It does not have to make electrical contact (it will help a bit but not the effort worth). Been there, measured it.

- At a good setup you probably will have a Z= 12+15=27 Ohms  (there it is I did make an estimation!) my point here:

- You will need a matching device / transformer (one toroid <500W, two or three at 1K5) to get from the guestimation value to connect it to your 50 Ohms coax feeder into the shack.

- There will be many other voices from other people having less/similar/more/different experience. 

My tips:

- Don't sweat it. At 1/8 vertical part, even with a sloping wire, one can make a good signal. Even at some crooked elevated radials.

- Read Low Band Dx'ing by ON4UN; it tells you how to do it, both in KISS mode as well as freaking nerd-mode. There will not be a great difference unless you have the hardware AND lots of real estate.

- Use an antenna analyzer which shows Xc, Z and SWR; the cheapest Rig Expert or any like is best in the field (no MFJ). I have had several different ones in the past 20yrs.

- Use a portable VNA if you must, but only if you have the experience how to use it. Or have someone come over to do that 😊.  No Smith chart crap; that's for nerds and NOT effective in the field, unless you use smith charts on a daily basis.

- Measure at the antenna, using a pigtail coax to about 0,5 to  1m length. Yes one can measure it in the shack using professional network analyzers/ VNA's etc.  when the feedline is properly calibrated out. Been there. A friend has a large HP in his shack.  In the end we prefer that small but very practical Rig Expert.

Happy experimenting!

73
 Mark, PA5MW

-----Original Message-----
From: Topband <topband-bounces+pa5mw=home.nl at contesting.com> On Behalf Of Gabriel - EA6VQ via Topband
Sent: Thursday, March 05, 2020 19:02 PM
To: topband at contesting.com
Subject: Topband: Inverted L with elevated radials for topband

I have some doubts about installing and inverted L with elevated radials for 160m.  I have been searching in Google and find some contradictory information, so I would appreciate very much if you can help me with your own experience.

The antenna would be supported by a 16 m (52.5 ft) high fiberglass pole placed on top of a 3m (10 ft) high small tower with the horizontal part of the L slopping down to a 5 m (16 ft) high mast about 23 m (75 ft) away.  Two to four tuned elevated radials can be placed, although they will have to be bent due to space restrictions.

Now the doubts:

What impedance can I expect at the feed point with this configuration?

How to match it in order to feed it with a 50 ohm cable?  Some pages say that no match is required, only a choke. Other pages say a hairpin is necessary as the impedance can be too low. So other say that a tuner at the
base is required?    I am confused L

What performance for DX can be achieved by this antenna?  Is it really good or do you have some better suggestion for a really small lot where no ground radials are possible?

Would it be worth to use a higher fiberglass pole, let's say 4 m (13 ft) higher, in order to lengthen the vertical section of the L?  Would the difference be noticeable?

Anyone has real experience setting up this antenna with elevated radials?
Most information I can find on Internet is related to ground mounted antennas.

Thanks in advance!

73. Gabriel - EA6VQ

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