bit of an extreme design: co phased vertical with base load

[Nationalpatriot]

I'm surprised no one mentioned the cancellation and directionality of the ½ wave spaced 'array'.

Maybe you already know this, but placing two antennas ½ wave apart puts them 180° out of phase so when you draw a line running from one to the other antenna you will have cancellation along the projection of that line in both directions, ie: N-S.
Perpendicular to that line, or 'broadside' to the antennas (E-W) you should have gain over only one antenna. This is called a figure-8 or cardioid pattern

If you wanted better omni-directional performance than just a single omni antenna, simply place them 9' apart, not 18'.

The length of the ¼ wave antennas, using ½" EMT should be right about 8' 6" for center-of-band 27.185mHz.

You need to find out the exact velocity factor of the RG-6 from the manufacturer then multiply it by 108" to determine the exact working length of each ½ of the 75ohm phasing harness for ¼ wave (9') spacing. For ½ wave spacing they will both need to be 3/4 wave long to reach the T connector.

I would strongly recommend using a T connector and SO-239 connectors with appropriate down sizing spacers for the RG-6.

If you cut a piece of RG-6 1¼ wave long for one side only you will effectively turn your array sideways and the cancellation will then be broadside to the antennas if they are 18' apart.

If you wanted to set it up for switchable 90° E-W or N-S performance it would require two SPDT relays and a DC relay switching voltage wire to select between the 1¼ and 3/4 wave pieces of RG-6 on one side only.

If you elevate the antennas so they are just high enough to allow a 30° down slope of ¼ wave long elevated radials for each antenna, (maybe 3' antenna base height) you will only require 4 radials each, and should perceive gain over a flat ground mounted system, plus your SWR will be closer to 1:1 as each antenna will be ~52ohms instead of ~37ohms.

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- Later you could add 2 more antennas, all 9' or ¼ wave apart in a square configuration, and build a switchable phasing harness for a "Four Square" antenna array and WOW, what a superb selectable directional!

Our 4 square for 80m at the cabin is simply amazing. Provides strong performance with signals which just aren't there on the wire or the Butternut vertical.

Hmm 3 feet is all it would take? I might be able to get that with the saw horses I'm using as bases, I think the stand area would be big enough for support. I AM wanting the figure 8 pattern but eventually might add the 2 other antennas

 

[W5LZ]

Don't get too 'hung up' on that 'cancellation' idea, nothing is being canceled at all. Redirected because of the fields relationship, yes. Change the phase relationship between those two antennas by 90 degrees and you just switched direction in which the radiation goes by 90 degrees. A very common way of changing directions with phased antennas with just a switch.
The distance between the two antennas can vary by quite a bit and they will still be directional, just less so. When you get down to about a 1/4 wave of separation or less, the resulting radiation pattern starts going towards a shape like a 'football'. It's still directional, but the 'nulls' to the sides aren't quite as deep. Lengthening the distance between antennas tends to make those 'nulls' deeper till you get to 1/2 wave length separation, where things start going into multiple lobes. There are common graphs showing the pattern changes with separation. They make a lot more sense than me trying to describe them.
Spacing can change the relationship between interacting fields, and phasing/timing can change things too. The two seem like they do about the same things but they don't really.
- 'Doc

 

[Nationalpatriot]

Thanks everyone for your help, I just (finally) got the EMT pipe cut to length, sanded down, insulated and holes drilled for the connection of the center conductor of the coax. (I managed to snap 3 drill bits too, maybe I was a bit too hasty trying to get them drilled.) In the picture below you can see the saw horses I will be using (and lining up conduit brackets from top to bottom to hold the thing, I think it'll work) just for added safety I will be running nylon rope between the two masts and from the masts to the ground just as guy lines.




Also, Rick sent me a good link for the graphs (if anyone else is interested in them I can send it to them) I understand a lot better what was being talked about

 

[jazzsinger]

Alrighty, I'm working on another new design (yeah, sorta obsessed with having multiple radio antennas but hey, I have a lot of Coax at a good price).
I have been looking over a few different ideas and finally have sorta combined one that i think in theory might work. *ahem cue the National Patriot graphics department*

best distance I've been reading for cophased is about 18 feet (1/2 wave length and the harness between the two antenna is to be 75ohm not the 50 that the antenna is made out of. Any guidance/deter would be great appreciated!

you would want the antennas at least 18 feet (1/2 wave) apart.

the cophasing lengths from each antenna need to be 3/4 wave long,but you must take int acount the velocity factor.you couldn't make them 1/4 wave as including velocity factor the rg59 at a 1/4 wave would only be around 6 feet on either side,which with 18 foot spacing would make it impossible to screw the pl plugs into the antennas.any odd multiple of a 1/4 wave can be used above 3/4,ie 5/4,7/4 etc.

having a 9 foot whip with a coil making up the other 1/4 wave to make the antenna half wave would be very lossy and would present matching difficulties,not to mention the fact the coil length wouldn't be 9ft.coils don't add the missing length,what they do is tune out the reactance of the antenna.

if you really want to cophase antennas (a lot of grief for very little gain in the real world) your best bet would be 2 x 1/4 wave gp antennas,which are easily fed with coax and require no special matching techniques,the sirio m400 stardusters would be ideal,but if you just want to use 9 ft whips then you can add 3 or 4 approx 45 degree downward sloping wire radials to both antennas which would give an even better match.


cdx 007,

putting two antennas 1/2 wave apart only gives 180 degree phase delay if one is fed with a half wave longer harness which would result in an end fire pattern with less gain over the standard broadside radiation pattern normal with 1/2 wave spacing and phasing stubs of the same length.

placing them 9ft apart may give more omni directional pattern but the gain would fall off sharply and give very little improvement over a single purely omni directional antenna.

in the real world there is very little to be gained from co phasing unless you switch in different lengths of phasing lines and understand how they work to obtain the maximum benefits in whatever chosen direction you want to work in.a hell of a lot of hassle and cost for minimal improvement,as cb's can generally hear much further than they can transmit too that money and effort would be better put into purchasing a good quality class ab linear amplifier,it also prevents your real estate looking like a gigantic hedgehog/porcupine.

 

[Nationalpatriot]

Got 'er done! works fantastically. Took me a little while to get everything together but not too bad.

^ the antenna at full. it's a little bit hard to see the other mast but it is up there I assure you

Incase anyone wanted to know how I connected the coax to the mast. I drill a hole through the pipe and used a 5mmx25mm bolt washer and eyelet connector on the coax (and then soldered and insulated the outer braid for the radials) for the bolt i first slid the bolt through, put a nut on, then the eyelet then a washer and then another bolt just to hold it in. Everything transmits greats and yeah, like you all said there is deep voids in the pattern.
I may not end up having guywires after all, they seem to be quite sturdy without the cables.

One other good thing about this design was that the 1/2" brackets are just tight enough so that I can slide the pipes up with a decent amount of effort, this has allowed me to only do 4 radials per mast but on the other hand has given me the ability to store this easily in my garage if I need to or if I ever wanted to transport it, I could toss it in the back of the truck no problem (with only about 10 inches of the mast sticking out of the covered box)

As for a name for the rig, I think I'll be christening it the "MEP-27/18" the MEP being for my late grandmother who passed away (Marry Ellen Pearl) and of course the 27 being the rough frequency and the 18 being the 1/2 distance

 

[Nationalpatriot]

Update: I know this thread is a bit old but just letting you know the whole setup is working great, I hooked up a switch to make it end fire and broadfire, as well as hooked up a switch to switch from the cophase to dipole antennas for better RXing. I am talking to people about 10 miles away with a standard radio!

 

[CDX-007]

Update: I know this thread is a bit old but just letting you know the whole setup is working great, I hooked up a switch to make it end fire and broadfire, as well as hooked up a switch to switch from the cophase to dipole antennas for better RXing. I am talking to people about 10 miles away with a standard radio!

Glad to hear you are pleased with the performance!

When you switch back & forth from end-fire to side-fire directly toward & away from a station, what amount of cancellation/change in signal strength are you getting on the S-meter? ( presuming you've already tried testing for that.)

I'm also curious what lengths of 75ohm you ended up using?

 

[CDX-007]

 

[Marconi]

CDX-007, I blame my lack of stamina and follow thru on my age, but I'm not too sure about you younger guys these days. Seems that the modern day CB'er just don't like sentences with "?" at the end.

What happened to your LW150 project?

 

[CDX-007]

Hi Marconi,

It's on hold as I might be getting a crank-up tower and it would make comparisons much easier!

I hope to have something done before the really bad weather gets here, but we've gone from 97° to 48° in 3 days and had a wind storm through here that knocked down our fence which I now must repair before I have any more antenna fun. :sad: On top of that I dropped my D-104 and ruined the element...

The transducer gods are against me!!!

 

[Nationalpatriot]

Glad to hear you are pleased with the performance!

When you switch back & forth from end-fire to side-fire directly toward & away from a station, what amount of cancellation/change in signal strength are you getting on the S-meter? ( presuming you've already tried testing for that.)

I'm also curious what lengths of 75ohm you ended up using?

Completely forgot this thread was open, just tested the array again few weeks ago, after about 10 minutes trying to figure out why "Doc" couldn't hear me very well 6 miles away, I realized I was on W-E not N-s, I adjusted my switches, keyed up and gained about 5db. I've got some fiddling to do to it but she seems near perfect. The change between end and broad fire I haven't measured, but it's amazing. I've skipped quite a few times with this set up, reaching from about lansing to grand rapids. I have to jump on a clear channel and make sure it's clear, but I can do it.

I'm working on a reflector array right now. I've spent the past 6 months collecting materials for it (I live on a chicken farm so I've got chicken wire out the wazoo, work at a farm business so I get scrap metal all the time, also got 300 yards of high pass wire to use for it)

 

[hotrod]

just get yourself a sirio topone antenna. its only 9feet in lenth
and was designed foe low height appications