4-bay VHF dipole array project

Status
Not open for further replies.

prcguy

Member
Joined
Jun 30, 2006
Messages
10,271
Location
So Cal - Richardson, TX - Tewksbury, MA
A single dipole will be close to 50 ohms, just attach coax and cut the elements to the best match at your operating frequency. Here is another idea if you don't need to cover the entire VHF band - use PVC pipe in place of the copper pipe and stick a simple wire dipole inside. You can tape the dipole to the outside of the PVC pipe durning tuning and stuff inside the PVC pipe after. You can even pull a length of coax center conductor through the braid and use the center conductor and braid as the dipole elements.
prcguy
how do you wire it if you only want one dipole?
 
Joined
Dec 24, 2004
Messages
787
Location
Emporia, KS
I have another question. What is the SWR on the antenna? If i build it i will need to Transmit on 2 meters. the documentation says 2:1 VSWR. isn't that kind of high or what does this mean? I have 2 similiar dipoles made out of 1/2 copper based off of a folded dipole DB antenna. though mine are regular dipoles. my swr is 1:5 1. I can't get it any lower. I thought it was a good match. But rechecking it, it appears higher. hope the paint isn't affecting it. I just have mine on PVC support arm/mast. should they be on a metal mast like yours then? I used PVC instead of metal but the same dimensions as the DB antenna. The arm is about 4 1/2 inch long like the DB antenna.
mine is now silverish dark gray not the green in the photo. http://picasaweb.google.com/soaked.jeans/RadiosAndAntennas#5052709223134096434

Eric Burris kc0ldt
 

kb2vxa

Completely Banned for the Greater Good
Banned
Joined
Mar 22, 2005
Messages
6,131
Location
Point Pleasant Beach, N.J.
Guess what? This type of antenna is about 40' high in itself and designed for mounting on the side of a tower or on an extension at the top. Once upon a time I used one on the side of a 50' tower which put the bottom dipole only 10' off the ground, "a real piece of work" being it's supposed to be way up on a much higher tower. Thank goodness it wasn't my station, I'm not known for installing grass burners. You'd do much better with a collinear which gives the same gain in about 10' above the radials.

"my swr is 1:5 1. I can't get it any lower."

Why would you want to? (;->) Here's the rub, each bay (dipole) affects the SWR and things mount up pretty quick. The trick is tuning each one individually IN PLACE which ain't easy because tuning it on the ground is one thing but it will behave differently once you mount it. Then you attach the phasing harness and pray it all comes together right, frankly I hate those darn things. BTW, I suppose you mean 1.5:1 now don't you? Now shut up you out there in the peanut gallery, just trying to be helpful with ratio problems and not being snotty... this time. (;->)
 
Last edited:

prcguy

Member
Joined
Jun 30, 2006
Messages
10,271
Location
So Cal - Richardson, TX - Tewksbury, MA
To answer the last two posts, the antenna I built in the documentation has about 1.2:1 VSWR at resonance and covers most of the VHF band including 2m at about 2:1 at band edges.

Each dipole is tuned individually with 50ohm coax on the ground attached to a similar diameter pipe that will be used for the finished antenna. I find no change in VSWR on the ground, in the garage or on my tower once the antenna is completed.

This type antenna is not 40ft long, its closer to 20ft at VHF and it can mount on two 10' TV masts or a 21' length of fence post would be ideal. With only 20' of pipe the bottom element will hang over the mounting support. The antenna without a mast could also mount to the side of a tower that is otherwise out of room for antennas.
prcguy
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,271
Location
So Cal - Richardson, TX - Tewksbury, MA
I forgot to comment on kb2vxa's statement "You'd do much better with a collinear which gives the same gain in about 10' above the radials". There is no collinear from any manufacturer that will give equal gain in the same length, a 10' VHF antenna will be hard pressed to pass the 3dBD omni gain mark. It takes about twice as many 1/2 wave elements in a collinear arrangement to equal the same gain as 1/2 wave dipoles at the correct spacing to provide maximum gain. a 21ft DB Products or Sinclair 4-bay dipole array is rated at 6dBdD omni gain and a 21ft Cellwave Stationmaster with 8 internal 1/2 wave collinear elements is rated at 5.2dBD gain. Same with all the other major commercial antenna manufacturers. Ham and scanner companies like Diamond, Comet, Create, Hustler, etc have very inflated gain ratings.
prcguy
 
Joined
Dec 24, 2004
Messages
787
Location
Emporia, KS
Thanks for the information prcguy. I have another question i forgot to ask. If i use 1/2 inch copper pipe how would that affect the SWR's, design, and reception? I was thinking of using 1/2 pipe as my old projects are. Plus 1/2 inch is cheaper and lighter. would the measurements be the same?

thanks again, Eric
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,271
Location
So Cal - Richardson, TX - Tewksbury, MA
The size of copper pipe I used in the article fits snugly inside the PVC Tees and I don't think there is an approproiate size PVC Tee for 1/2 copper. The dimensions will probably change slightly.
prcguy
Thanks for the information prcguy. I have another question i forgot to ask. If i use 1/2 inch copper pipe how would that affect the SWR's, design, and reception? I was thinking of using 1/2 pipe as my old projects are. Plus 1/2 inch is cheaper and lighter. would the measurements be the same?

thanks again, Eric
 
Joined
Dec 24, 2004
Messages
787
Location
Emporia, KS
Is this design required for the dipole to be on the metal mast for good swr's? Reason why i ask is i have a dipole out the window on a 2 ft PVC support arm with no metal mast on the antenna. the arm is mounted directly to the wall mount. Another dipole is on a PVC mast for portable base station use. those designs are much like yours but on 3/8 inch pipe not the 1/2 i stated earlier. and the measurements are copied off of a Decibel folded dipole antenna and cut to around 17 3/4 inches. 3/8 inch tubing matches a 1/2 aluminum tubing.

I'm thinking the metal mast is part of the matching device. What if the top element is about the metal mast?

sorry for so many questions. I'm just learning the ins and outs of these designs.

Eric Burris kc0ldt
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,271
Location
So Cal - Richardson, TX - Tewksbury, MA
You can make a single dipole and match it to 50ohm coax if you keep the coax at 90deg from the dipole for a few feet. It will have 0dBD gain but reasonable band width. If your making a phased dipoe array on a metal mast the top element should be below the mast by the amount shown in the article. The elements for a phased dipole array are tuned individually with 50ohm coax while on a mast.
prcguy
Is this design required for the dipole to be on the metal mast for good swr's? Reason why i ask is i have a dipole out the window on a 2 ft PVC support arm with no metal mast on the antenna. the arm is mounted directly to the wall mount. Another dipole is on a PVC mast for portable base station use. those designs are much like yours but on 3/8 inch pipe not the 1/2 i stated earlier. and the measurements are copied off of a Decibel folded dipole antenna and cut to around 17 3/4 inches. 3/8 inch tubing matches a 1/2 aluminum tubing.

I'm thinking the metal mast is part of the matching device. What if the top element is about the metal mast?

sorry for so many questions. I'm just learning the ins and outs of these designs.

Eric Burris kc0ldt
 
Joined
Dec 24, 2004
Messages
787
Location
Emporia, KS
prc guy,
I was wondering if you could help me with phasing harness? I want to make a 2 dipole array instead of 4 because of mast space. 4 is too big and i wouldn't know how to mount something that big and the landlord is already concerned that 3 antennas is too much. I am using my previous built dipoles. they are aluminum equivalent to 3/8 copper pipe. i think it's 3/8, smaller then 1/2 inch. The support arm is 4 inches from center of element to the end of pipe, not counting the T connector to the mast. What would the dimensions be for the harness? would you build it for me if i can't do it? I have some RG59 but i think it's all the foil shield type. without buying the commercial brands how would i know the velocity factors and stuff? Radio shack doesn't list those specs nor would anything from walmart.

I was wondering since the commerical antennas have metal support arms do i have to use PVC on mine? I don't know if it matters. I put mine on my mast outside a few feet of the ground and tested it. flat match on ham band. i cut the support arm to around 5 inches instead of 6 to match that of my other one that was copied from the DB ant and the SWR went from a flat match to 1:5:1 in the middle of the 2m band and it was shifted up the band a bit. So i think it would stretch closer to the RR band that way. i'm not sure how to figure out the spacing to the mast without guessing.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,271
Location
So Cal - Richardson, TX - Tewksbury, MA
A metal or PVC sopport arm doesn't matter as long as you tuned the individual dipoles to frequency with 50 ohm coax and it was attached to a metal mast while tuning. Most current CATV type RG-59 is foam type and will have a similar velocity factor to what I used in the article, so the phasing harness dimensions in the article should be close and you would only make half the harness for two elements.

The metal mast is a reflector and part of the antenna. Each dipole becomes a 2-element beam in this case and that's why its directional when all elements are arranged on one side. If you don't use a metal mast the coax will interact with the pattern and that would require a lengthy study to see the effects.

The spacing of the elements to the mast will affect several things including the pattern, efficiency and impedance. I used a common spacing measured from several commercial antennas and if you change it very much your on your own.

To make a proper harness for a 2-element version you would use PL-259s and a T adapter so the antenna is fed with 50 ohm coax. I don't have time to solder connectors but if you were planning on using 75 ohm coax I could make a harness in about 5 min using T&B Snap-N-Seal connectors and an F type T adaptor for your coax interface. I would leave it to you to strip the ends and make the dipole connections and I don't to start a harness making business for everyone on the forum. I have never tested the antenna with 75 ohm coax and can't guarantee the result.
prcguy

prc guy,
I was wondering if you could help me with phasing harness? I want to make a 2 dipole array instead of 4 because of mast space. 4 is too big and i wouldn't know how to mount something that big and the landlord is already concerned that 3 antennas is too much. I am using my previous built dipoles. they are aluminum equivalent to 3/8 copper pipe. i think it's 3/8, smaller then 1/2 inch. The support arm is 4 inches from center of element to the end of pipe, not counting the T connector to the mast. What would the dimensions be for the harness? would you build it for me if i can't do it? I have some RG59 but i think it's all the foil shield type. without buying the commercial brands how would i know the velocity factors and stuff? Radio shack doesn't list those specs nor would anything from walmart.

I was wondering since the commerical antennas have metal support arms do i have to use PVC on mine? I don't know if it matters. I put mine on my mast outside a few feet of the ground and tested it. flat match on ham band. i cut the support arm to around 5 inches instead of 6 to match that of my other one that was copied from the DB ant and the SWR went from a flat match to 1:5:1 in the middle of the 2m band and it was shifted up the band a bit. So i think it would stretch closer to the RR band that way. i'm not sure how to figure out the spacing to the mast without guessing.
 
Joined
Dec 24, 2004
Messages
787
Location
Emporia, KS
A metal or PVC sopport arm doesn't matter as long as you tuned the individual dipoles to frequency with 50 ohm coax and it was attached to a metal mast while tuning. Most current CATV type RG-59 is foam type and will have a similar velocity factor to what I used in the article, so the phasing harness dimensions in the article should be close and you would only make half the harness for two elements.

The metal mast is a reflector and part of the antenna. Each dipole becomes a 2-element beam in this case and that's why its directional when all elements are arranged on one side. If you don't use a metal mast the coax will interact with the pattern and that would require a lengthy study to see the effects.

The spacing of the elements to the mast will affect several things including the pattern, efficiency and impedance. I used a common spacing measured from several commercial antennas and if you change it very much your on your own.

To make a proper harness for a 2-element version you would use PL-259s and a T adapter so the antenna is fed with 50 ohm coax. I don't have time to solder connectors but if you were planning on using 75 ohm coax I could make a harness in about 5 min using T&B Snap-N-Seal connectors and an F type T adaptor for your coax interface. I would leave it to you to strip the ends and make the dipole connections and I don't to start a harness making business for everyone on the forum. I have never tested the antenna with 75 ohm coax and can't guarantee the result.
prcguy
not sure what those snap-n-seal connectors are but i am interested. how much would you charge? Would you include the Pl259 with it? I'm kind of short on those.
thanks,
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,271
Location
So Cal - Richardson, TX - Tewksbury, MA
I can easily make a 2 dipole harness that terminates with an F type T adapter but I don't have time to make one with PL-259s. You can feed it with 75ohm TV coax and take a small hit on VSWR or get a male F to SO-329 adapter to use 50ohm coax. The F type T adapter is about $3.50, Snap-N-Seal connectors are about $.35 ea, so I could make you a 2 dipole harness for about $5 + shipping. You will have to strip and prep the ends with lugs to connect to your dipoles.
prcguy
not sure what those snap-n-seal connectors are but i am interested. how much would you charge? Would you include the Pl259 with it? I'm kind of short on those.
thanks,
 
Joined
Dec 24, 2004
Messages
787
Location
Emporia, KS
I can easily make a 2 dipole harness that terminates with an F type T adapter but I don't have time to make one with PL-259s. You can feed it with 75ohm TV coax and take a small hit on VSWR or get a male F to SO-329 adapter to use 50ohm coax. The F type T adapter is about $3.50, Snap-N-Seal connectors are about $.35 ea, so I could make you a 2 dipole harness for about $5 + shipping. You will have to strip and prep the ends with lugs to connect to your dipoles.
prcguy
So i can use 50ohm coax for this? I thought i would have to use 75 or is that just as with 4 bays only? If we go 75ohm then would you include the Pl-259 if i put it on myself? I'd hate to use a F-pl259 adapter cause of signal loss. I'm excited to play with this antenna. btw i have a single dipole built like yours out the window now with lightweight aluminum from an old antenna for the mast.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,271
Location
So Cal - Richardson, TX - Tewksbury, MA
Each individual dipole is tuned for 50 ohms and the common feed point for the entire antenna is 50 ohm but the phasing harness is all 75 ohm coax. You can use 75 ohm coax to feed the entire antenna if you don't mind the additional loss and VSWR.
prcguy
So i can use 50ohm coax for this? I thought i would have to use 75 or is that just as with 4 bays only? If we go 75ohm then would you include the Pl-259 if i put it on myself? I'd hate to use a F-pl259 adapter cause of signal loss. I'm excited to play with this antenna. btw i have a single dipole built like yours out the window now with lightweight aluminum from an old antenna for the mast.
 
Joined
Dec 24, 2004
Messages
787
Location
Emporia, KS
Each individual dipole is tuned for 50 ohms and the common feed point for the entire antenna is 50 ohm but the phasing harness is all 75 ohm coax. You can use 75 ohm coax to feed the entire antenna if you don't mind the additional loss and VSWR.
prcguy
i see want you meaning on the adapter and loss. you were talking about going 75ohm all the way to the radio. My scanner right now has 75ohm with the ST-2 on it.

If you can send me the PL-259 with the cables i can put that part on myself. Unless you have an N connector. I have an xtra LMR400 cable that also needs a connector on one end as it is bare. It has a N female on the other which hooking to a jumper cable would eliminate a coupler. that'd be sweet!!! tell me how much and i'm ready to send some money your way.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
10,271
Location
So Cal - Richardson, TX - Tewksbury, MA
If I'm going to "make" the harness I would tune it and that requires having the connectors installed. I can make and tune an entire harness in minutes using F connectors (have hundreds on hand) but PL-259s take more time to properly install than I am willing to spend right now and I only have a few PL-259s left. If you have the ability to solder on your own PL-259s the rest is just some RG-59 foam type TV coax. The coax lengths listed in the instructions should be close for most foam type RG-59 coax. I thought you didn't have the ability to solder or install connectors and that's why I offered.
prcguy
i see want you meaning on the adapter and loss. you were talking about going 75ohm all the way to the radio. My scanner right now has 75ohm with the ST-2 on it.

If you can send me the PL-259 with the cables i can put that part on myself. Unless you have an N connector. I have an xtra LMR400 cable that also needs a connector on one end as it is bare. It has a N female on the other which hooking to a jumper cable would eliminate a coupler. that'd be sweet!!! tell me how much and i'm ready to send some money your way.
 
Joined
Dec 24, 2004
Messages
787
Location
Emporia, KS
If I'm going to "make" the harness I would tune it and that requires having the connectors installed. I can make and tune an entire harness in minutes using F connectors (have hundreds on hand) but PL-259s take more time to properly install than I am willing to spend right now and I only have a few PL-259s left. If you have the ability to solder on your own PL-259s the rest is just some RG-59 foam type TV coax. The coax lengths listed in the instructions should be close for most foam type RG-59 coax. I thought you didn't have the ability to solder or install connectors and that's why I offered.
prcguy
I'm sorry for the misunderstanding. I looked at RS website and they don't even have RG59 that is solderable. So i'm not sure if it's easy to come by locally. Then i've have to order online and then order connectors and all that. I don't even know what F T connector to use as RS site only shows splitter and diplexers. I didn't realize you were going to test it. we can do it your way if you still want.
 
Status
Not open for further replies.
Top