I'm trying to tune up a pair of MFJ 20M hamsticks fashioned as a horizontal dipole using their dipole mount for that purpose. Using an MFJ antenna analizer, I can tune it to a 2:1 SWR (at 14.2mhz) at about 15ft off of the ground. I think I'm getting it so short that the whips are interfering with the loading coils of the sticks and affecting the SWR, preventing me from getting any lower.) However, I've built a current balun with about 8 turns of a pair of 14ga insulated wires through a ferrite torroid. When I put it in place about 8-12" under the antenna my SWR goes up to about 4:1 The balun housing is plastic with SO239 connectors on each end. In each case. I have about three feet of LMR400 between the analyzer and the connection to the antenna (leaving at 90 degrees from the dipole) to get me away from the antenna. Any idea why the SWR gets thrown off so much?
Current balun throws off SWR
- Thread starter bharvey2
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The leads heading out of the balun to the whips become part of the radiating antenna and can add length, otherwise what core material did you use? For a 1:1 choke balun its usually easier to make your 8 or 10 turns with small coax rather than parallel wires since the lowest impedance you can usually get with parallel enameled wire is probably at least 100 ohms.
prcguy
prcguy
The balun housing is terminated with SO239 connectors at both ends so I have the hamstick dipole, PL259 right angel adapter RG58 x 8" jumper with PL259 ends, the balun housing and a 36" length of LR400 with PL259 connectors leading to the analyzer. The parallel wires wrapped around the ferrite torroid are not enameled but rather #14 stranded THHN. There is probably 1 1/2" of pigtails on each end of the torroid soldered to the SO239 connectors. I was told the the torroids are type 43 but as they are second hand, I'm not sure how to verify that.
I definitely notice some RF introduced into some powered speakers near the transmitter when the current balun isn't in place. - Clearly some common mode action going on.
As an aside, one thing I noticed with the hamsticks in the dipole configuration: as the whips are inserted further into the fiberglass section and approach the coil, there is a very quick dip in SWR. However, placing them too far in ( making the coil an iron core rather than air core) the SWR jumps up rather quickly. As the whips approached the coil, the SWR could get down to around 1.2:1 but at too low a frequency. I used some other whip stock I had that was much shorter (athough the overall hamstick length was the same" but could never approach the low SWR. Just an observation.
I definitely notice some RF introduced into some powered speakers near the transmitter when the current balun isn't in place. - Clearly some common mode action going on.
As an aside, one thing I noticed with the hamsticks in the dipole configuration: as the whips are inserted further into the fiberglass section and approach the coil, there is a very quick dip in SWR. However, placing them too far in ( making the coil an iron core rather than air core) the SWR jumps up rather quickly. As the whips approached the coil, the SWR could get down to around 1.2:1 but at too low a frequency. I used some other whip stock I had that was much shorter (athough the overall hamstick length was the same" but could never approach the low SWR. Just an observation.
K7MEM
Member
According to this article on Hamstick Dipole 20m, you should be able to get the SWR down to about 1.2:1 using the MFJ 20 Meter ham sticks. But the article also says that tuning is very touchy.
What are you using to support the dipole? With the dipole being short, and only being 15 ft above ground, a metal support pole could be interacting with the dipole elements.
What kind of core are you using for the current balun? The wiring that you are suggesting is not a 1:1 current balun. Two parallel wires wrapped around a core several time make a 4:1 or 2:1 balun, depending on how the wires are interconnected. You need three wires to make it 1:1.
The link below shows a 2:1 current balun. The text contains a link to a picture of the innards.
Ham radio blog by OZ1BXM: Homemade 1:2 balun
The link below show the construction of a 1:1 and 4:1 current balun. Note that the 1:1 balun needs three wires. I use a similar balun for my dipoles but I use a ferrite rod rather than a core.
EA4EOZ, an amateur radio electronic enthusiast: Build your own HF balun
Martin - K7MEM
Right now the dipole is mounted on a long 2" x 4"clamp to my open wood garage door. The balun design is this one:
https://www.google.com/url?sa=i&rct...ua5uWoIhA1QHPSrBHZ-FiPhw&ust=1463781465163538
Since the core is second hand, its provenance is questionable. That is what I'm leaning toward.
https://www.google.com/url?sa=i&rct...ua5uWoIhA1QHPSrBHZ-FiPhw&ust=1463781465163538
Since the core is second hand, its provenance is questionable. That is what I'm leaning toward.
This is where I looked up the different types of baluns and the 1:1 current balun is the one described on this site:
VK5AJL - Why buy baluns - make your own.
The three wire one doesn't make sense to me as being a 1:1. Is the drawing an error?
VK5AJL - Why buy baluns - make your own.
The three wire one doesn't make sense to me as being a 1:1. Is the drawing an error?
A 1:1 current balun is typically made from coax or two parallel wires wrapped around a ferrite core and one wire is considered hot and the other ground. Sounds like there are no excessive wire lengths so it would take some experimenting to find why its having so much affect on the match.
Usually a full size 1/2 wave dipole in free space or multiples of a half wavelength above ground will be higher than 50 ohms at resonance and probably around 70 ohms. Shorten the elements with loading as in a Haystack and I believe the impedance will be lowered some. Place the horizontal dipole close to the ground and the impedance is lowered further and possibly close to or less than 50 ohms.
A 1:1 balun made of parallel wires around a large core probably uses about 10ft of wire and its a transmission line of at least 100 ohms in my opinion. 10ft of 100 ohm transmission line operating on 20m and in line with an antenna close to 50 ohms and an additional feed line of 50 ohms could make a messy match at some point.
prcguy
Usually a full size 1/2 wave dipole in free space or multiples of a half wavelength above ground will be higher than 50 ohms at resonance and probably around 70 ohms. Shorten the elements with loading as in a Haystack and I believe the impedance will be lowered some. Place the horizontal dipole close to the ground and the impedance is lowered further and possibly close to or less than 50 ohms.
A 1:1 balun made of parallel wires around a large core probably uses about 10ft of wire and its a transmission line of at least 100 ohms in my opinion. 10ft of 100 ohm transmission line operating on 20m and in line with an antenna close to 50 ohms and an additional feed line of 50 ohms could make a messy match at some point.
prcguy
K7MEM
Member
No, there is no drawing error. In fact, the link you provided shows the same three wire design under the section on "1:1 Transformer Baluns". While the link you provided indicates that there is a core used, the design can be used with no core at all. But a ferrite rod core helps make it more broad banded. Here is a link to a picture of the three wire 1:1 balun used as the center for a dipole.
http://ve3elb.ham-radio.ch/projects/balam.jpg
I find these type of 1:1 baluns at ham fests all the time, for about $8. They usually being sold because the owner doesn't know that they really are. The one I am using with my Trap Dipole, I found in the garbage, when I was working in Germany in the early 80s. I cleaned it up, tested it, and have been using it ever since. I have two or three more that I keep on hand for other antenna projects. I like them because of the built in strain relief, when used as a center insulator. I think Unidilla still makes a 1:1 and 4:1 in this style.
Well then I assume the balun is constructed correctly. However, the core is still in question. Have you actually tested the balun by itself? There should be some information in the MFJ antenna analyzer manual on how to determine whether your balun is working as you planned.
Martin - K7MEM
I haven't had a chance to check the balun by itself. But, I am now suspicious that the core isn't what i though it was. I'll have to read up on the instructions on the analyzer to determine how to check for proper balun performance. Also, since the balun effectively doubled the SWR, I wonder if the design I used wasn't actually that of a 2:1 balun. Thanks guys for your help. It's much appreciated.
Karl-NVW
Member
First step is to dfecide which one antenna parameter you are going to optimize by tuning. You get to pick from: Insertion Loss, Resonant Frequency, Reflected Power, Maximum Forwqard Gain, Mounting Height, Maximum Power Capability, Main Lobe Take-Off Angle, to name just a few. PICK ONE and ignore the rest, then go make some contacts. After over 50 years in Amateur Radio I have yet to find any antenna where all those can be aligned to optimum with a single adjustment.
Karl-NVW
Member
A true balun (Balanced to UNbalanced converter) will NOT have SO239s on both ends. That's an UN-UN and on many the two shields are connected, not isolated by the core windings; thus it can't do what you need for this application. THIS is where you need the core with triple winding, one for the left side Hamstick, one for the right side HamStick, and one for the unbalanced coax transmission line. Pay very careful attention to where each end of the three windings is connected and do NOT connect the T-Line shield to the shell odf either antenna connector.
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