160 m. ground system, Z and earth changes

[K5UJ]

Not sure if this is technical enough for the technical section but I noticed something interesting to me at least with my inverted L and ground system.  I feed it with a 70 foot run of LDF4-50 and  tune it in the shack.  I mostly park around 1880 so I don't have to adjust the L network tuner in the shack much.  The other night I got ready to go but to my dismay noticed the vswr had changed dramatically.  I was able to achieve an acceptable match to 50 ohms but with a significant change in the tuner (inductor always in series, cap grounded) -- I had to flip the cap to the input side of the inductor, use about half the amount of L I had before and so on....   I was hesitant to dump a lot of power into this, thinking there might be low resistance short to ground somewhere on the feedline, or some other antenna interaction problem had surfaced.  The 20 mw from the swr analyzer usually isn't enough to blow through any corroded short and I have occasionally had the unpleasant experience of having 300 w. suddenly complete the connection (or open it) providing me with a brief light show  in the tank circuit before I kill the carrier.  

I decided to do some testing to make sure.  to make long story short everything checked out (I have two feedlines going out there so I can swap them to check them out) and I had to conclude that the sudden hard freeze we had changed the ground system enough to alter the feedpoint Z.  I sort of remember using the current tuner configuration last year so I guess this is an annual winter thing that I had forgotten about.

I'm wondering if anyone else with monopoles and ground systems on 160 notice a fairly big change in winter when the ground freezes really hard.

I should also mention that I am down in a river valley near the bank of a river where the ground (black dirt) is almost always moist (when it isn't frozen) so the ground conductivity is usually pretty good. 

Rob

[DMOD]

We just had some new snow pack and a another dip in temps here in Iowa and the impedance on my Inverted-L for 160 changed significantly.

Unfornunately, and until I get the Viking I up and running, I can only run 20 Watts AM.

Phil - AC0OB

[WD8BIL]

I had to retune mine also Rob. Not much. Just a tweek on the output cap. My tuner is a T match and it's at the base of the L. You shud consider moving the tuner to the antenna! or not !! 

L's are low Z anyway. On a typical L/C tuner the inductor SHOULD be on the low Z side. I wonder, once the ground unfreezes ifn you'll still be able to leave it the way you got it now?

The way you had it would have needed almost twice the inductance.

Remember; HI Z = cap side    LO Z = ind. side in a L/C tuner.

[k4kyv]

L's are low Z anyway. On a typical L/C tuner the inductor SHOULD be on the low Z side. I wonder, once the ground unfreezes ifn you'll still be able to leave it the way you got it now?

That depends on the length of the L.  If the total length is close to a quarter wave or less, the Z will likely be low, so the capacitor needs to be on the input side of the inductor.  If the total length is somewhere close to a half wave, the Z will probably be higher than 50 or 75 ohms, so the capacitor will need to be on the output side of the inductor.

[WQ9E]

I feed my 18HT "Hy Tower" as an inverted L on 160.  I stretched the horizontal section so it is a little long (inductively reactive) and I use a series capacitor at the feedpoint to tune out the inductive reactance and this provides a good match to 50 ohm coax.  My capacitors are made from lengths of RG-11 and I have 5 of them, selected via a BC-375 "tap switch" from Fair Radio, that allows me to cover the band with low SWR. 

[WD8BIL]

That goes without saying, Don. Knowing Rob's setup is what prompted that advice.

Most inverted L users I run across run 1/4 or 3/8 wavelength. But as you point out, halfwaves are a different beast!                     

[KM1H]

Those are several good reasons for elevated radials, I dont even own a tuner

Carl

[Bill, KD0HG]

I have never seen any significant base impedance change on broadcasting verticals from summer to frozen ground in winter. BC stations almost always incorporate an impedance bridge to measure R and Z, measured regularly, and they barely change. *maybe* an ohm at the most.

Besides, the current on a short vertical is between the radiator and the wire part of the ground system, intervening earth has very little to do with it unless the vertical's ground system is really, really poor or radials are buried many feet down.

The conductivity of the dirt under a vertical is always going to be way less than that of any radials in the ground system. Current doesn't flow back to complete the circuit through dirt, wet, dry or frozen dirt like it does through a few highly conductive wire radials.

If a major impedance change is occurring from winter to summer or from wet to dry, it's a sign that the antenna's ground system could use significant improvement.

Bill

[flintstone mop]

Bill's post explains why BC stations put so much effort into the transmit plant.

They will stay on the air without flinching from the WX.

And I'm sure with these fluctuations in freezing temps, and the next day it's 50 F, that stations running high power RF, better have their dehydrators running and lines pressurized with nitrogen............OR POOF!!!

A million watt (erp) UHF station in Washington DC, of all places, had to replace their transmission line coz of real cheap maintenance procedures.

[K5UJ]

Bill and others, thanks for the comments.  Looks like I'm going to have to go out when the wx is better and do some work.

I have plenty of copper sheet.  Time to do some brazing I guess. 

Rodger, that idea with the switchable caps is a good one.  I know about the deal with a motorized variable cap out there.  I think I need to lengthen the L wire to get that to work.  Currently I think it's 90 degrees at around 1840. 

[Steve - K4HX]

I not sure this is correct or I'm misunderstanding what you are saying. A good ground radial system is even more important for a short vertical since the R_rad is lower.

Besides, the current on a short vertical is between the radiator and the wire part of the ground system, intervening earth has very little to do with it unless the vertical's ground system is really, really poor or radials are buried many feet down.

[K5UJ]

that's what I thought--the more the difference between the antenna Z and the ground system R the higher the efficiency.   This only pertains to radials on the ground or buried--I think elevated antennas are a completely different design situation.

[Bill, KD0HG]

I not sure this is correct or I'm misunderstanding what you are saying. A good ground radial system is even more important for a short vertical since the R_rad is lower.

Steve-That's what I was trying to say... ;-)
In the past, I have tried to calculate the Rrad of a given vertical, say an example calculates to 20 ohms.  Then I have matched the vertical with a tuner to 50 ohms. There is an RF ammeter at the base of the vertical  and a wattmeter  in line with the transmitter output.  Set the transmitter up to be producing exactly, say, 100 watts.  Then read the RF current into the antenna at 100 watts. Calculate the apparent feedpoint Z of the antenna, using  the 100 watt reference, if you know the power and the current you can calculate the antenna’s base Z.  If this example comes out to measure 75 ohms, then you know that the ground resistance is 75-20 ohms, or 55 ohms. If the antenna Rrad is reasonably close the calculated 20 ohms, then you know the efficiency of the antenna is about 36%. Then it becomes easy to modify or improve the ground system; instead of guessing, you know exactly how much improvement you are making in the ground system.

In short, it’s all the same as the olden days of ham radio, strive for the most antenna current (Fire in the wire!) for a fixed power input.

Bill

[k4kyv]

Another way of looking at it is to think of the entire radial system as a "shield" to isolate the vertical radiator from the lossy earth.  Yes, I know some have argued this is not a valid analogy, but this is more a matter of semantics and helps explain the point.  Ideally, the radials would be lying right on top of the ground, where there is nothing between the vertical radiator and the radials, so they can perform their function of returning the displacement current as ground current back to the base of the vertical, without intervening losses. The only reason to bury radials is to provide physical protection from ground traffic such as human foot traffic, animals, vehicles, lawnmowers, etc.  If they are buried to a shallow depth, there is negligible lossy earth between the ground plane and the vertical.  But if they are buried too deeply, that begins to defeat the whole purpose of the radials, since a significant amount of lossy earth is inserted into the displacement current path between vertical radiator and ground radial. If the radials were buried deeply enough, nearly all the displacement current (which takes the form of ground current) would be returned back to the base via the soil, loss and all, and very little ground current would appear in the radials.

With a short vertical, keeping the ground  losses as low as possible is even more important than with a full size quarter wave or longer. And with buried radials or radials lying on top of the ground, there is no "resonant" length.  You don't need a set of 125' radials for 160 interleaved with 65' ones for 75 and 30' ones for 40, although such a configuration might be an effective way to use a limited amount of copper since you would have greater radial density near the base of the vertical, where the ground current would tend to be higher.  But this would be the case for the entire frequency range of the vertical, including 160m.

OTOH, with elevated radials, resonant length does become a significant factor, and quarter wavelength radials for specific operating frequencies makes the ground plane more effective.

[Steve - K4HX]

OK Bill. I'm tracking and you're explanation is excellent. Sometime people forget that the equivalent ground resistance adds to the feed point resistance. Most verticals anywhere near 0.25 WL tall with a good ground system should not come close to matching 50 Ohms.

The radiation resistance of a vertical can be approximated by the following formula.

R_rad = 36.6 x [(1-cosL)²/sin²L]

where L is the electrical length of the vertical in degrees.

[Bill, KD0HG]

I used that technique when I had an inverted L, and it greatly surprised me at what sort of radials helped, or didn't.

I tried adding a zig-zagged 1/4 wave radial to the base of the L and the base resistance went up, of all things. So I removed it. I then tried rolling out an X some 50' of 1" mesh chicken fence at the base, and it lowered the base Z- A lot. So it stayed. I added a 8' ground rod, it made no difference at  all. Added gallons of soluble salts to the ground rod, it still didn't do anything.The best ground return that I found was attaching to my neighbor's steel cyclone fence, which ran around his lot, not mine- LOL.

Don, yes, I agree, I'd take six short but straight radials over three longer 1/4 wave jobs.

An effective vertical can use more real estate than a humble dipole for the same efficiency. Not just a pole in the ground.


Bill

[W9BHI]

I am using a ZeroFive 43 ft ground mounted vertical with 2500 ft. (fifty 50ft) of radials and an AD5X designed matching network for 80-160 meters and it works very well on 160.
I don't seem to notice any changes in SWR when it rains or snows.
I live in the middle of a corn field with good soil, maybe thats why.
Just my $0.02.
Don W9BHI