The AM Forum

Here is my delima, I am on a city lot of 60'x120'.  I have one of two choices for antenna's for 75M, a Inverted Vee or a 75M Loop.  The Vee would be maybe 50' to the apex and the loop would average around 35'. 
Which would be better for me? 
I am wanting to be able to make it back to the left coast, and hopefully talk out east.  Of course it will be cut for 3885 and tune down to 3870 for the west coast AMI net.
Thanks for the help in advance.

Richard

Richard,

For 75M, the inv vee is your best bet.   A full wave 75M loop couples to the ground more than a inv vee (losses) and at 35' will be a cloud burner for locals only, to boot.

Try to get the ends of the inv v up as high as possible. The ideal textbook dipole is straight, flat and as high as possible, between 60' - 130' high, depending on the type of takeoff angle you desire.  In addition, a center supported inv vee is easier to keep up in the air.  Unless you add more elements, the single, simple dipole is the best antenna you can have for HF work.

T

don't know where you're located.  50 x 100 foot lot here.   put up dipole center fed 130' long & let ends dangle down.
feed with ladder line and KW Matchbox.  You could maybe get away with 15' on each end hanging down? 

you want to get back to west coast, from where?  East I presume.   best bet is 1/4 w. vertical base fed with lots of radials.
I use that plus aforementioned dipole.   Last winter late one night I worked a guy in Seattle area w. dipole and switched to vertical and gained 20 dB.  I'm just west of Chicago.

Rob

I use a loop but it is in the vertical plane.  It's held up by two tall trees.  The top edge is up about 85ft at one end and about 50ft at the other end (the 50ft end was much higher, but the 100ft oak tree holding that end fell over).  It is fed with a 75ohm 1/4 wave matching coax which is connected directly to 50ohm coax.  Feed point is along the lower edge which is about 12ft above ground.  My antenna seems to work well.  It is above a stream bed and nearby wet terrain which probably helps.

If you can't set up a full wave loop like I described, I would take JJ's advice and go with an inverted V with the center feed point up as high as possible.

Fred

For 75 I regularly use a full wave horizontal loop at about 40 feet.  Working into the east and west coast along with the deep southeast has not been an issue here from the Midwest but if I were on the W. coast wishing to regularly work into the E. coast l would go with a vertical and the best ground system I could fit to the real estate. 

Sometime I will have to run some A/B tests on my Hy Gain Hy Tower (with extensive radial system) versus the loop for stateside.  The major use of the Hy Tower has been for 160 and 80 meter SSB/CW DXing along with checking into a local 160 meter net. 

Most of the antenna's radiation is from the current portion. A 1/2 wave inverted V will radiate a lot of the signal near the feedpoint. If low angle is desired, go with the current portion as high as possible. Flat tops are nice but not everyone has a way to do that. Just avoid small included angles. Ends can be bent to make up length.

loops near the ground suck been there wasted my time

Thank You all for your answers.

I should have said that I am located in Mustang, Oklahoma which is just 10 miles from the Oklahoma City Limits.
I moved from San Diego which I was a regular on the AMI net out there.  I was running a inverted Vee out there at 70' and had good luck with it.
Since moving out here I have been fighting the city to be able to put up a 50' tower, long story short it took the ARRL laywers and ARRL Division Manager to get it done. So now I am ready to start getting my station up and running again. 
The snag that I have run into is the city will not let me put a antenna up past the front of my house, so that rules out a straight dipole.   I may go ahead and try it, what can they do but tell me to move it lol, or as you guys have already stated put up the Vee.
I figured the loop would be to low to the ground, but it would be the easiest to install here.
I need the most of what I can make so that I can try and keep the SSB boys down in Texas at bay lol, they love to interrup the West Coast AMI Net.


Richard

Consider an inverted L, then. If the total length of wire is near 1/2 wavelength, the point of max current will be near the highest point of the antenna. Think of it as an end-fed inverted vee. You can go 50' vertical with a mast or tower and out to the rear of the property with the horizontal portion.

Richard,

Since the loop is the easiest for you to install, try it and if it doesn't cut it for distance then try a different antenna.  I put up the loop as a fairly temporary antenna while I ran low loss coax from the vintage station in the house to my Hy Tower in the pasture and 7 years later I am still mostly using the loop for AM.  It is far quieter on receive than the vertical and I have no problems working out long distance.  I run the legal limit on AM and I have no trouble making it through the "pileup" on the first Wednesday AM nights to the NCS. 

For receive, if the SSB stations in cause major problems consider a small separate receive loop to null them out.  These small loops are very directive and I have used one in the past to get rid of a noise problem.  Using a receiver that allows you to separately receive either sideband on AM will also go a long way towards helping you with QRM. 

Rodger, since I have a bunch of wire I may put up the loop and may try out the iverted L at the same time and do a comparison test to see which is better for my needs.

Now as for the inverted L which I completely forgot about as a option. Can you run the ground side to the tower which has a Radial ground system attached to it?  What effects would it have?  Also my radial system is attached to a chain link fence which surrounds the house.

Richard,

If your tower has a radial system already you could shunt feed it as a vertical for 160 and 80. 

If you go with an inverted L, I wouldn't advise having the vertical part of the L close to and parallel with a grounded tower.  I use my Hy Tower in an inverted L configuration for 160 using a homebrew solution instead of the power limited Hy Gain methods.  The end of the horizontal section attaches to the tower holding my 4 el quad and I cut the L so it is slightly longer than resonant on the bottom end of 160.  This way I am able to feed it through switch selected series capacitors (made of RG-11 coax) which provide a proper match across 160 by canceling the inductive reactance.

Rodger, 

Doesn't the tower have to be isolated from ground to feed it?  I know enough about antenna's to make me dangerous lol.  How would I shunt feed it?  I will do some more reading on it and see if I can answer my own question lol.

Inverted V would be my choice on that small lot.  Open wire line, fed to a balanced tuner. Dont do anything dumb like running a balun and then coax into the house.   ::)   Length does not matter much.  Use alot of wire if you can. What ever fits the lot.  A guyed off military mast would work. I have fiberglass but these guys turned me on to the aluminum ones which are much stronger.  I use Trueladder line dot come for my open wire line with copper weld wire for the top section. 

I think the real key to this is to get as much wire as you can way up in the air.  I had a thread like this a few weeks ago and that kinda hit home.  Once the wire is out in the clear is when you are really going to enjoy it.

Mine is 45ft flat top in pine trees.  I was coast to coast on wednesday night.  Cali to Long island.   

C

I also have a 60' x 120' suburban lot.  For 40 - 10, I use a chimney mounted on a one story house 4BTV with 16 radials (4 per band).  From Cleveland, with about 500 watts, I regularly work all over the world with good reports.

For 75 and 160, I load up a 130' inverted L.  The vertical part is at the back of the lot, and is only about 25' high.  The horizontal part is about 100' long, and is anchored at the front of the house.   I also get great reports on 75, and "OK" reports on 160. 

Because of so much experience in AM broadcasting, I am very nit-picky about grounds.  To make antennas like mine work, I try to connect as good of a ground as I can achieve at the antenna feed point, and match the antenna to transmission line with a simple L-C network.  That ground is VERY important to making the "L" work.  I have a half dozen radials on the ground, but I also have 100's of feet of chain link fence tied to it as well.  I have to conclude the fence REALLY helps.

Good luck!

73
Ted  W8IXY

No.  There are a few variants of GROUNDED vert's built out of towers in the ARRL books over the years...  I was just reading on in the ARRL Antenna Compendium, but can't recall which issue it was.

You could also add a drop wire from the top of your tower, run it away with some decently long spacers, and feed the tower as a folded dipole, for lack of better ways to describe it before I've had my coffee.

If you HAVE a good radial system, then the Inv L is the way to go.  I put one up a couple weeks ago, and I'm very happy with the results.  I use a remote tuna at the feedpoint, so am currently limited to 150 watts PEP, but it works fine business for me...  I've even made it into a couple AM nets with 15 watts of carrier with it.

Mine has 4 radials at 10 or so feet, and a 40 foot vert section (I'm using my 40 foot tower to hold it with a large standoff at the top and a piece of wire for tensioning... I get winds > 95 MPH here and ice load can be measured in feet.) with a 67 foot horizontal section.  It tunes pretty much everywhere...  I do have a problem with "RF in the shack" above 20 meters, but I've got it to where it only disables my mouse during voice peaks.  Could be the tuna and feedpoint is 10 feet from the operating position...  ANOTHER reason for non-qro.

I'll be putting other antennas up, just picked up an 80 foot tower, and I've got 11 acres at 5800 feet :), but I hope this gives ya an idea.

The Inv L LOVES radials.....  It's a top loaded vert!

--Shane
KD6VXI

Ok.... after doing some reading on shunt fed towers, I think I may tackle it and see how it works.  This way I won't have the city or neighbors after me by adding more stuff in the air for them to complain about  :-X.

From what I have read all I have to do is top feed my 50ft tower which has a 10M beam on top at 55FT with a 43ft long boom. Space the hot feed line around a foot from the tower isolate it going down to the ground, add a Vaccuum tuna, tune to lowest swr and start talking lol.  I am sure it is not that easy but let me see what happends  ;D.
Thanks again for all your input.

Richard

Richard,

Most of the antenna books cover this and there are probably internet resources on shunt feed systems but if you cannot find the info PM me and I will scan some stuff for you over the weekend.

Rodger

I ran that way for a while with my old tower.  You likely wont be happy unless its DX your after.  98% of the time, The dipole beat the thing.  On 160, It worked decent and put out a big signal. Be sure to report back on how it goes, Good luck.

C

Clark,

When you say DX how far out are you talking about?  What I am looking for is to be able to talk to the West coast and East coast from here in Oklahoma City.  There are no locals that talk here so I am not worried about that.  A straight dipole is out of the question do to limited space.

Richard

You didn't have NEAR enough ground radials I bet.

--Shane

When I lived on a city lot, an inverted L worked terrifically for me.
Couldn't use an inverted vee as I had no front yard support for one end of a true dipole.

I used the chain link fence surrounding the lot as a counterpoise.

55' or so vertical and 60 or so feet horizontal to a mast on the roof of the garage in the alley.

I did try a 70' dipole fed with open wire line and a tuner, the inverted L smoked it by far on 160-80-40.

I had plenty of radials. The Inverted L worked well. But under 800 miles the dipole always beat it.  I had a 65 ft tower up loaded and it too worked well but the dipole won on day to day use talking to the guys 500 miles or less.


C

If you have all these hassles in a place like Mustang, Oklahoma then we're all screwed.  May as well get ready to operate AM in some simulated internet VOIP mode.  BTW I was in Calif. last week and managed to get outside at night a couple of times with a small sw bc rx and copied K6EC one night in Riverside calling CQ on 3870 with a real strong strapping signal.  Never heard him back east. 

At my previous QTH I lived on a 50 x 100' lot and just wrapped the dipole around the perimeter.  Actually, it was a sort of W configuration with the center near the center of the lot.   Worked for me

Al

Ok... I have my mind set and I am going to shunt feed my tower.

I went to my storage unit yesterday and got a 500ft roll of 10 gauge wire to add some more radials to my towers grounding system.  The towers guy system is no problem since I have phillystran cables in place.  As I mentioned before it is a rohn 25 at 50ft with a 43ft boomed 10M beam at around 58ft on a hazer system.  I have the Hazer grounded to the tower and I am working on grounding the mast to the tower for added measure.

The question I have is.... If I use my MFJ 259B to find the 50 ohm feed point on the tower for the shunt line, Can I hook up the center conducter of my 1/2" Heliax to the shunt line and of course the ground to the shield, and be able to use my Nye Viking Tuna? . This system will be used with a Henry 2K so I want to make sure everthing is done right so things don't go up in smoke.  Or would it be safer to add a vaccuum variable out at the base of the tower?

Richard

If you found the 50 Ohm point, you wouldn't need a tuner.

Be careful with all cables running up the tower including rotator cable. Attach coax shield of beam to the base of the tower. Bypass all rotor cable leads to the baes of the tower. You will fry the rotor cable and maybe the rotor if you do not bypass the leads. It might be a good idea to bypass the rotor cable leads at the rotator. All cables running up the tower lege will get RF coupled to them when you shunt feed the tower and if they are not terminated to the tower could generate some high voltage RF due to the length. You can assume zero volts at the grounded base and very high voltage above the shunt connection.

The 7/8 Heliax to the beam I have grounded at the base of the tower and at the entrance to the shack so that is taken care of.

How would I bypass the rotor cable? This is all new to me, so I am in stupid mode right now lol. I was just planning on disonnecting the rotor from the control box when I was going to transmit, but I see what you are saying.  What do I need to do to bypass it?

Richard

.01uf disc ceramics at the highest voltage you can find. I bet a common mode choke at the rotator would also be a good idea to keep RF out of the innards. Tom Vu played with shunt feeding so maybe he can add some ideas.
I would terminate the 7//8 in a few places to keep it from becoming a radiating extra element.

Ok... I will break the 7/8's up as well.  How big on the voltage should I go on the caps?  I think I have a bunch of 5k lying around, I will have to make sure though, is that high enough or will the voltage need to be higher?

On the rotor cable I am guessing that you ground all eight wires?  If so I will do that prbably at the top and bottom just incase.  I have a weather station on the tower as well so I will ground those leads as well while I am at it.

Sorry for all the questions, Just would rather ask then learn the hard way by burning something up.


Richard

I think they are very good questions. You want the tower leg voltage the same as the cable voltage. as you go up the tower voltage increases until you hit the quarter wave length.