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Antenna Guru Needed




 
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WV Hoopie
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« on: July 02, 2005, 11:42:11 PM »

I have a small city lot problem and can't afford the earth stretcher. So here is the story.

First I put up a 40 meter dipole fed with 600 ohm open wire (#14 with 5 inch spreaders), a shade over 64 feet in length (the dipole) and about 25 feet in height. Worked fine: other than there is little 40 meter activity here in the Pacific Northwest. Can just barely here some midwest AM carriers at about 7.2925 MHz. Might make one or two contacts a week.

Lots more AM activity on 75 here on the north left coast, plus mexifornia AM stations: So I've added two loading coils to make a short 75 mtr dipole.  Same feed line, its about 41 feet in length. Over all length of the dipole is 64 feet 2-3/8 inches or there abouts. Coils are about 22 feet 5-5/8 inches from center, another 9 feet 7-9/16 inches to the ends. Coil dimensions: Dia. 4-1/2 inch, length 3-1/4 inch, 25 turns of #14 and a Dia/Pitch of 0.5, about 61 uH. Most equations call for 58 uH, but a little bit more is better. I can short out a turn or two if needed, damn tough to add more wire later.  Found a web site using HamCalc, by George Murphy for the dipole numbers and picked one close to the most efficient. Used another program to design a coil with the least amount of heat loss, using materials at hand.

Antenna tuner is the 1976 univeral transmatch, a dual-section cap 200pf per section in parallel with a 28 uH roller inductor , a series 350 pf cap, and a 1:4 baluns (three T-200-2 cores). I think the caps are rated at 3000V.  Transmitter: Johnson 500.

Problem Below

So now the tranmitter loads up in CW, I can switch to AM and key the mike, with the audio gain on the RF deck at 0, no problem other than no one would hear me. If I increase the audio to 1, where it usually runs, (1 or a shade more will give 100% mod on the scope) the least amount of modulation will trip the PA overload relay. Transmitter is fine on a dummy load.

So, I can hear only relays tripping. PA cathode current meter never has a chance to move, steady at 250 mils. Could I be on the edge with the tuner? Modulation is just enough to cause an arc? I can't detect any burn, pitted, black, or discoloration in the tuner. Or, guess I can try after dark, maybe the coils are arcing? Don't know if I would see the coil arc?

Gentlemen, any ideas?Huh?

Gotta get on 75 and the neighbor would be P***ed if I hung wire in his yard!

Tnx,
wd8kdg
Craig
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K1JJ
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« Reply #1 on: July 03, 2005, 11:18:15 AM »

Hi Craig -

The key is that it works at full power into a dummy load and not
the antenna. You said the overload relay trips when the TX is
modulated... what kind of transmitter, T-368 or something?  

Maybe one of the guys who has the same rig can determine if
they are prone to RF getting into them and tripping falsely.
[from the anttenna RF field]

You seem to be indicating that the X4 peak power of modulation
is doing it. Try this: Load the TX up to 25% of its normal carrier
[if you can] and then modulate it about 75%. see if that trips it.
This will be below your previous power that DID NOT trip it. So,
if it trips, then you know it is audio related and not raw power.

Then load it back up into the antenna, full carrier power. Watch
the scope and SLOWLY increase audio to 10%, 20%, etc and
study the scope. You will see the beginning of slight arcing on
the scope at some point that will look like fuzzy spikes, if the
antenna or something else is arcing.

I suspect it is RF getting into the tripping circuit. Try adding
ground cables (thick braid) to the TX, antenna tuner, audio
gear, at various points. Tie them together well and try moving
the ant tuner around to a different spot farther from the TX.

Do you have another rig or linear of similar power to try to
see if anything arcs? Watch it on the scope.

Report back what you find and we will go from there.

You want to find more symptoms to when this happens so we
can guess some more... :-)

73,
Tom, K1JJ

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Fred k2dx
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« Reply #2 on: July 03, 2005, 11:51:35 AM »

Hi Craig. You say the mic gain is usually on '1'? Sounds like you are getting RF into the mic input, I would think it has more gain than you need if you set it at 1.

The advice on grounding is important, use a common point for all the equipment, don't daisy chain it.

Try keying the tx without the mic attached. If the rig will not trip the relays as you advance the mic gain, try RF bypassing/choking the input. Maybe a 2.5 mh rf choke in series with the mic jack, bypass both ends of it making a pi  type filter.

If the rig trips without the mic attached, look for RF getting back into the tx, either in an audio stage or a protection circuit. It could be on an  unshielded control line or something. How close is the antenna? You have moved the tx effectively half the distance from the antenna by going to 75 meters. (longer wavelength). Maybe relocate the antenna? With balanced feed worrying about resonating the antenna isn't an issue so don't be concerned about getting to the coils. You can bend those end sections into ANY shape or direction and it won't matter. They don't have to be symetrical either....treat them as end loading capacitance, they don't radiate enough to be concerned about.

Have you tried changing the length of your balanced feedline? Look up the older ARRL handbook tables. You may try lengthening it, electrically on 75 it is half the length it is on 40.

73,
Fred   k2dx
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WV Hoopie
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« Reply #3 on: July 03, 2005, 11:59:15 AM »

Hi Tom,

The transmitter is a Johnson Viking 500. As stated, it does load fine on all bands into a dummy load.

Being able to load the antenna in the CW mode with no problems, only having the PA overload trip when modulation is applied in AM has me puzzled. I did have the daughter, who still has 20/20 vision, watch the loading coils last night for arcing. She didn't see any flashes, doesn't mean there isn't something going on with the epoxy holding the windings of the loading coils.

I'll try different power levels, but I think its audio related. All it takes to trip the PA current overload relay is a very slight finger movement on the screen of the D104. I can just barely see waveform start to appear on the scope.

The grounding is via #10 copper wire tied to the Tranny and receiver (no ground lug on the tuner) going to a water well and short ground rod on the side of the shack. If I can't figure this out by Tuesday, I know where I can purchase some copper braid and add another "longer" ground rod at a different distance from the shack.

So, today I'll add a ground lug to the tuner, and the idea of moving the tuner away from the tranny sounds great and will try that also.

Tnx,
Craig
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WV Hoopie
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« Reply #4 on: July 03, 2005, 12:25:00 PM »

Quote from: Fred    k2dx
Hi Craig. You say the mic gain is usually on '1'? Sounds like you are getting RF into the mic input, I would think it has more gain than you need if you set it at 1.

The advice on grounding is important, use a common point for all the equipment, don't daisy chain it.

Try keying the tx without the mic attached. If the rig will not trip the relays as you advance the mic gain, try RF bypassing/choking the input. Maybe a 2.5 mh rf choke in series with the mic jack, bypass both ends of it making a pi  type filter.

If the rig trips without the mic attached, look for RF getting back into the tx, either in an audio stage or a protection circuit. It could be on an  unshielded control line or something. How close is the antenna? You have moved the tx effectively half the distance from the antenna by going to 75 meters. (longer wavelength). Maybe relocate the antenna? With balanced feed worrying about resonating the antenna isn't an issue so don't be concerned about getting to the coils. You can bend those end sections into ANY shape or direction and it won't matter. They don't have to be symetrical either....treat them as end loading capacitance, they don't radiate enough to be concerned about.

Have you tried changing the length of your balanced feedline? Look up the older ARRL handbook tables. You may try lengthening it, electrically on 75 it is half the length it is on 40.

73,
Fred   k2dx



Hi Fred,

The mic is a plain jane D104, no amp. I've wondered about never having to set the audio gain on the Viking 500 any higher than 1 to get 100% modulation. Don't have another 500 to compare it with, maybe someone else can jump in with info on  their 500 audio setting.

Will also try disconnecting the mic and advancing the audio gain to see what happens. RF getting into something is prime suspect. Relocating antenna or changing the length of the feed line will be tough. To get 64 feet of antenna I'm using the longest length of the back yard, no where else to place the antenna. The antenna is hung from a tree on the back corner of the lot and a wooden mast on the roof of the house. Can't shorten the feed line but a few inches, if I lengthen it outside of the shack, it will be at neck level if you know what I mean.

Tnx,
Craig
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WV Hoopie
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« Reply #5 on: July 04, 2005, 12:50:59 AM »

Really wanted to increase the distance between the transmitter and tuner, but didn't have enough wire and or copper braid for grounding.

So test one: Leaving the transmitter in the tune position the PA cathode current was at 90 mils and resonant. I could advance the audio gain and reach 100% modulation on the scope. What a strange waveform, sort of rounded tops with a short whistle tone. The over current relay did not trip.

Test two: I moved the switch to the operate position and didn't increase the antenna loading, just resonated the plate tuning. The PA cathode current was at 170 mils. The over current relay tripped with about 20% modulation.

Test three: Loaded the transmitter up to 250 mils. This is where I usually run the transmitter, 50 mils under spec. House voltage is higher than when these were built and I don't have a variac yet. With the mic disconnected and in the AM position, I could run the audio gain full clockwise and the relay would not trip.

I think I see an issue with the mic cord. The twisted pair inside of a shield;one is used for audio, the other for PTT. The PTT line should be outside of the shielding, audio only inside. The PTT line does have chokes near the mic jack and where it leaves the RF deck and where it enters the PS/modulator, but that interconnecting cable isn't shielded. The PTT is connected to the over current relay.  Hmmmmmm  Going shopping for different mic cord on Tuesday, maybe some copper braid.

Going to play with it tomorrow a little more, any suggestion?

Tnx,
wd8kdg
Craig,
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« Reply #6 on: July 04, 2005, 09:44:45 AM »

Snip the ptt lead inside the mic connector and try it again using the knife switch.
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K1JJ
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« Reply #7 on: July 04, 2005, 01:28:17 PM »

You're on the right track, there, Craig.  No doubt you will
find it now. RF feedback/ rectification.

If the intended rewiring and shielding doesn't solve it, consider
sprinkling in some .001 disc bypass caps into those
PTT/overload areas to the ground/braid strap..

The fact that the rig runs FB at a reduced power level
and 100% modulation tells us that it's a level thing.

On many of the higher bands, I wud have to slowly ramp
up the power, fix an RF feedback thang, ramp up higher,
fix another... until I cud run full power on 10M/6M, etc.  
It's really no different on 75M, just different circuits and
things to tame.

Let us know.

T
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Favorite Song - Trololo thru the years:  
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WV Hoopie
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« Reply #8 on: July 04, 2005, 03:01:51 PM »

Played radio this morning. I moved the tuner (no ground wire), its now on an ice chest and two 4X6's. This shortened the open wire lead in from 3-1/2 feet to about 8 inches. If there is any unbalance in the antenna system, good chance there is, the open wire will radiate.

Anyway, at full PA load 250 mils, I can reach about 50% modulation. So, no doubt RF is getting in. After lunch the PPT gets whacked in the mic connection.

later..........craig,
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WV Hoopie
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« Reply #9 on: July 05, 2005, 09:37:42 PM »

Well its been fun, passed the time and kept me out of trouble. So after all the tweeking, moving, checking bypass caps, and shorting out turns on the loading coils, this is the results.

Short antennas are a pain in the backside, as some things improved others didn't. As I was able to increase loading and modulation %; the roller inductor in the tuner got hotter faster.

Think the next thing to try is running wire around the house, trees, yard and get enough in some shape where it will not cancel itself out, plus load up on 75 with out arcing in the tuner.

Tnx for the suggestions,
wd8kdg
Craig
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« Reply #10 on: July 05, 2005, 10:07:52 PM »

I did a lot of experimentation with a short antenna last winter. Had a 120' center fed flat-top working on QRO AM on 160. It can be done. You will lose some power but that depends on how much copper goes in the important places. Commercial dog crap tuners, brown crap ladderline and smoking loading coils won't make it. You can get on 75 with your existing skyhook length. Lose the inductors, QRP stuff. Build a double balanced tuner. Check Rich Measure's site for an example. That style tuner will deliver power where the other's need the feedling trimmed or won't play at all when you turn up the gas. Use heavy homebrew feedline like #8 with plexiglass spreaders made from plexi window replacement stock. THHN wire passes through tight fitting holes in the rectangular plexi spreader. Glue into place with 2 part epoxy.
All heavy stuff will be able to handle high SWR on the feeders and low radiation resistance of antenna. You can save big bux by winding your own pair of inductors in the tuner. Roller inductors are not a must. Wind a pair using copper refrigerator ice maker water suppply copper pipe side by side ala bifiliar on a piece of PVC pipe. Unscrew one out of the other when done winding and you have a pair. You will need a vacuum variable to handle circulating current.
I've got pictures and should write all this up sometime.
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« Reply #11 on: July 06, 2005, 01:33:46 PM »

Quote from: Dave Calhoun W2APE
I did a lot of experimentation with
a short antenna last winter.  Commercial dog crap tuners, brown
crap ladderline and smoking loading coils won't make it.  Lose
the inductors, QRP stuff.  Glue into place with 2 part epoxy.

 :lol:  :lol:  :lol:

You're gagging me, Ape!  But so true.

BTW, is epoxy UV proof, or do you need to find a special mix?  
I need to use some to position the spreaders of my gay 6M
openwire feed system.  

T
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“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #12 on: July 06, 2005, 03:21:06 PM »

Good question. It seems to hold up. I've had the same homebrew feeders for the last 6 or 7 years. Used JB Weld 2 part epoxy.
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Ed/KB1HYS
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« Reply #13 on: July 06, 2005, 08:41:02 PM »

that JB weld is good stuff!!  once used it to repair an engine block that got damaged when the head gasket blew through...  ran on for another 1200 miles or so!!!   :shock:
I've kept some handy ever since.
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73 de Ed/KB1HYS
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« Reply #14 on: July 06, 2005, 09:56:08 PM »

Quote from: K1JJ
BTW, is epoxy UV proof, or do you need to find a special mix?  
I need to use some to position the spreaders of my gay 6M
openwire feed system.  T


There are two types of JB Weld.  One is quick setting and the other is the slow type.  The quick set type doesn't have the high temp properties of the slow kind.  On an antenna, that probably won't matter but on parts subjected to a lot of heat it could.
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« Reply #15 on: July 07, 2005, 11:35:13 AM »

Quote from: Ed/KB1HYS
that JB weld is good stuff!!  once used it to repair an engine block that got damaged when the head gasket blew through...  ran on for another 1200 miles or so!!!   :shock:
I've kept some handy ever since.


 Yep. Great stuff. Used it on a exhaust leak on a manifold pipe. Sealed it for a good year or so before it went to the dump.
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« Reply #16 on: July 15, 2005, 12:55:47 PM »

The Valiant 500 likes to see a very High Impedance on it's stock mic input. If you are only running the level at #1 position you may be running one of those pre-amplified D-104's? If so, bypass the amplifier & the transmitter will be happier. You can bypass it in the base of the mic.
Keith
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WV Hoopie
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« Reply #17 on: July 15, 2005, 01:54:50 PM »

Quote from: WA1HZK
The Valiant 500 likes to see a very High Impedance on it's stock mic input. If you are only running the level at #1 position you may be running one of those pre-amplified D-104's? If so, bypass the amplifier & the transmitter will be happier. You can bypass it in the base of the mic.
Keith
WA1HZK


Hi Keith,

Wonder no more, Just an old plain jane, no pre-amp, no place for batteries, crystal, D-104 microphonium.

But I still think its strange to run the audio gain at 1 or slightly more; antenna, dummy load, no difference. Oh well, the scope says 100% and if I hit it a little harder, the freq counter starts jumping to other frequencies.

Just the same, I've been eyeballing the back yard and pacing off distances. Going to try a full wave loop, will have to wind it around, up and down, up and over the roof, etc. Should be able to get about 260 feet of wire out there some place. If it loads up, radiates (cloud burner), and I can make some contacts: good.

A full size anything has to be better than heating up loading coils on the antenna or in a tuner. Using the 40 mtr. dipole in the back yard fed with homebrew 600 ohm open-wire, the only thing that even begins to get warm (after a couple hrs of QSO) is the baluns, three stacked T-200-2 cores.

So, its off to the hardware store for a couple more parts n' pieces.

later..........craig,
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