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eHam Forums => Antenna Restrictions => Topic started by: N2LWE on February 03, 2010, 11:59:36 PM



Title: 40 meter dipole works well on 80 and 20m
Post by: N2LWE on February 03, 2010, 11:59:36 PM
I finally put up my 40 meter dipole (Alpha Delta DX-40) up in my tree about 30 feet straight up the trunk of the tree with RG-8/U coax connected to the SO239 on the center insulator straight into the shack. I decided not to use the ladder line since a friend had suggested to leave it the way it was meant to be. Its installed as an inverted V. To my surprise I'm getting good reports not only on 40 meters but also on 80 meters and 20 meters. I've heard that this dipole should also work well on 15 meters although I've yet to accomplish that. I use a MFJ-969 tuner which works really well. Amazing how I had a MFJ-1798 vertical that was falling apart that originally cost $300.00 and never really performed well on 40 or 80 meters and now just a simple wire antenna for under a hundred bucks and I'm back in business.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: KH6AQ on February 05, 2010, 06:17:23 AM
While you are making contacts the radiated power on 80 and 20 meters might be only 10 watts due to the high VSWR.

A NEC simulation with the coax having 1 dB of matched loss shows additional loss of 8 dB on 80 meters and 11 dB on 20 meters.

Feeding the antenna with ladder line would reduce the line losses significantly.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: K3AN on February 15, 2010, 03:37:27 PM
Don't be fooled by the low SWR reading you get once you adjust your antenna tuner. The SWR on the coax from the tuner to the antenna is still very high on 20 and 80. On those bands you're sending 100 Watts into the coax, and the antenna is probably getting only about 10 Watts to radiate. The other 90 Watts is just heating the coax.

Yes, your 40M dipole will work quite well on 15, where the SWR will be relatively low.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: WB2WIK on February 15, 2010, 05:48:43 PM
Just thought I'd toss in one more agreement with the last two posts.

"Everything works," it's all a matter of degree.

A coax-fed 40m dipole is an awful antenna on both 80 and 20 meters, but it can certainly make contacts even if 90% of the power is lost in the coax.

Just think how much better it would work if that weren't the case!

If you have the room to do so, you can add parallel elements to the 40m dipole: Two more wires for 80m, and two more wires for 20m, and just tie them off to different end points than the 40m dipole uses, so there's some separation between the wires.  Then, you'd have a real tri-band antenna with much higher efficiency and you'd work MUCH more on both those bands than you can work now!  Maybe $10-$20 worth of wire, insulators and rope...


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: N0JEF on February 19, 2010, 05:13:10 PM
Heating the coax?  That is why he should of used the ladder line to begin with.  Your friend told you wrong.  Use the ladder line then your dipole becomes a doublet and will be a fine multiband antenna.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: N2LWE on February 22, 2010, 06:09:28 PM
Thanks for all the advice. When the weather gets warmer I will do what all of you recommend. Just for the record though, I've now made contacts on 80, 40, 20, 17, 15, and 12 meters. Unbelievible. When I had my vertical up I was never able to make contacts on 80 meters and rarely made them on 40 meters. So hard to believe with a couple pieces of wire what could be done. Anyway, I will make the change when the weather gets better. Thanks again for all the advice.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: N0XE on February 26, 2010, 02:12:36 PM
Get some good books on antennas, antenna theory and even  working DX, don't just always rely on things you migh find on the web or what others tell you, some advice is just great but some can be (not so great)  as well. Read and Read some more, ask questions, research and bottom line don't be afraid to experiment as well.  In the long run you will learn a lot more about antennas and different options and things you can try to put out a good signal. There are a lot of variables when it comes to antennas.  Verticals have their place, a good vertical with proper radial system  or counterpoise can be an excellent antenna, the angle of radiation tends to favor DX contacts, many of the big guns on 160 and 80 meters use Verticals for transmitting and then some other type of antenna for receiving as the man made noise tends to be worse on a vertical. Getting the most out of your vertical takes more then just setting in on the ground and hooking up a feed line.  this is where the term antenna farm comes in to play, the more options you have for changing conditions or locations you wish to work the better.  there is no one perfect antenna for all situations , what works great for you at your paticulair location many not work for me. I agree though with most of the other posts, change to ladder line on your feed line  and things will even improve. Steve is right, it does not take much to make a contact, especially with the high tech rigs of today, Qrp ops with a few watts, and even miliwatts at times can work the world so it is not that surprising your making contacts on 20 and 80, but think how much better your signal could be by not losinig that extra power as heat.  Good luck and keep the RF flying, most of all have fun with ham radio.  73 Jim N0XE


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: N2LWE on February 28, 2010, 04:03:01 PM
Thanks for the feedback. Like I said when the weather gets warmer I will be changing to the ladderline. Just amazed that when I had the vertical up 160, 80 and 40 meters were out of the question. Otherwise, the other bands were just fine. Now my latest on this 40 meter dipole just a few minutes ago I worked 160 meters. Something I have actually never done before since I've been licensed. So off of this piece of wire I've now worked 160, 80, 40, 20, 17, 15 and 12. Just waiting for 10 meters to pick up and I will have covered them all. Off a piece of wire. So, changing the feedline to ladderline could only get better. By the way, I am having a great time.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: N2LWE on February 28, 2010, 04:08:20 PM
Forgot to mention, in the near future I'll be brushing up on my CW. Then I'll be going after 30 meters. Then I'll have them all covered. Lol. I guess better late than never. I've had the CW bug lately and want to improve since I truly haven't focued on it much since I took my General back in 1992. I'm guilty. But like I said better late than never.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: N2LWD on January 22, 2011, 01:21:18 PM
I know this is an old thread, but I found it so I figure others might. :)
My question is: I want to feed my 40 meter dipole with 450 ohm ladder line. How long should the ladder line be? I am now feeding it with 50 ohm coax and the SWR is fine. And it gets out and receives fine. I have heard/read that I will be able to use it on 80 meters also. My 105' G5RV will not load up on 80 very well. I feed my home made G5RV with 30' of 450 ohm ladder line. Should I use the same lenght for the 40 meter dipole? I am not worried about 10/15/20 Meters as I also have a Hy-Gain TH3 Mk3 tri-bander.
Almost forgot, I have an antenna tuner in line...
Thanks for the help,
Sal, N2LWD


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: WB6BYU on January 22, 2011, 07:14:20 PM
Quote from: N2LWD
...I want to feed my 40 meter dipole with 450 ohm ladder line. How long should the ladder line be?


Long enough to reach from the antenna to the tuner.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: VE3QJ on March 15, 2013, 12:20:38 PM
i often hear people say "dont run ladderline in through window opening as that would be a fire hazzard".
your saying use ladderline as a feedline from the antenna to the tuner?
i would really like to know what to do.
thanks :-\


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: WB6BYU on March 15, 2013, 07:10:33 PM
Quote from: VE3QJ

...your saying use ladderline as a feedline from the antenna to the tuner?



That's the most efficient way to do it for multiband operation, unless you
happen to have a specific set of dimensions that gives a low SWR on
coax on some bands (such as the G5RV.)

Whether or not it is a fire hazard depends on the power level, the impedance
at various points on the feedline, and what you use for insulation.  I've taken
a lot of liberties with such installations and haven't seen any sparks yet
at 100W.  In one case I wrapped an old sock around my twinlead and closed
a metal window frame on it.  Sometimes I didn't bother with the sock.  I've
used two separate wires fed through holes in a vent screen, or a pair of
coax feed-throughs side-by-side through a wall.  If you have single pane
glass on your windows you can glue foil to each side to make a pair of
coupling capacitors in series with the feedline.

I'd be much more careful at higher power, of course, but I don't think
I've ever failed to find a way to get a parallel-conductor feedline routed
into the tuner in the shack when that was what I was planing to use.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: AC4RD on March 16, 2013, 06:13:50 AM
i often hear people say "dont run ladderline in through window opening as that would be a fire hazzard".
your saying use ladderline as a feedline from the antenna to the tuner?

I've done that for many years, running twinlead from the antenna into a tuner at the shack, with a clear fairly straight shot from the tuner to the window.   One way I've often done it is with various types of high-density foam, like the stuff that printers and computers have packed around them in their shipping boxes.  NOT styrofoam, but the squishy stuff. 

Doing it this way, you need two strips each slightly wider than the window opening, and anywhere from 1 to 2 inches square.  One strip goes at the base of the window opening, twinlead runs over that, second strip of foam goes between the twinlead/first strip and the bottom of the window.  Bring the window down hard enough to compress the foam, and use a small woodscrew on each side to hold the window in place.  Works fine, keeps the cold out (at least here in the sunny south), zero problems!   GL!


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: NO9E on March 27, 2013, 05:51:01 AM
I used to have a 40m dipole fed by a ladderline. Just 30 feet up. Worked coast to coast in 160m contest with 400W.

When traveling I often use  a tv flat cable from wires outside to the inside. The cable is either crashed by the door or by the window. Never a problem at 100W. Also, not a noticeable difference when working outside or inside.

Ignacy, NO9E


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: KF7DS on March 31, 2013, 10:49:58 PM
I have the same antenna that I plan to hang this month and was thinking about using 450 ohm twin-lead too.

The antenna will be at 35 Ft on it's north end and about 60 Ft at the south end, and mid-point at about 45 ft.....feedline drop to ground is about 45 Ft and then another 50 Ft to a remote antenna switch.

Where should I change from twin lead to coax, when the twin lead hits the ground?

The only way to the switch is on the ground, but won't this detune the twin lead or is it ok to run along ground surface?

What adapter would I need to change from twin lead to coax at or before the switch (coax 8 port switch)?

Any other issues I should consider (e.g. attachment at antenna)?

Thanks
Don KF7DS
Portland, OR


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: WB6BYU on April 01, 2013, 09:57:11 AM
If you are just using it on 40m, run coax all the way from the antenna to
the switch/shack.

If you want to use it on 20m and other bands, the proper adaptor between
the ladder line and coax is a balanced antenna tuner.

Unless you carefully choose the lengths of the antenna and ladder line to
give a low SWR relative to 50 ohms, a combination of coax and untuned
open wire line can cause significant losses in the coax due to high SWR.
It isn't difficult to lose half you power (or more) in moderate lengths of
coax with such a system.  For example, while a ladder line length of around
45 to 50' may give a low SWR on 20m with a 40m dipole, the SWR on 40m
and resulting efficiency will suffer.  It's quite difficult to find such a
combination that will give low coax losses over a range of bands, especially
if you have more than 50' of coax involved.  If you do use a remote balun with
coax to a tuner in the shack, I recommend that you keep the coax short -
inches rather than feet if possible.

Or use very low loss coax - this would have been a good use for some of that
RG-17 that I got rid of at the Salem Hamfest.  That might have kept your
efficiency over 50% across multiple bands.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: KF7DS on April 01, 2013, 09:35:24 PM
Good feedback and good to know. Bought it for 40m and will play with it that way.

Thanks much,
Don KF7DS


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: N0NZG on April 03, 2013, 11:26:41 PM
proof qrp works.


Title: RE: 40 meter dipole works well on 80 and 20m
Post by: VE3FMC on April 04, 2013, 04:45:22 PM
proof qrp works.

True enough, I have a 80 M coax fed dipole that tunes right up on 160 meters. Doesn't radiate with a damn but hey the radio sees a 1:1 SWR.  ;D 6 meter 3 element beam will load on 20 meters too!

Hey I have made contacts on my old Heathkit Cantenna that is sitting on the basement floor and that thing has a 1:1 SWR on all bands.  ;)