This probably isn't the right forum, but the knowledge probably resides here anyhow.?

I bought an Emtech ZM-2 to use with my QMX. I'm presently trying to use it with a 40m dipole (with current balun) fed by coax. The dipole is very resonant for me on 40m. With the tuner I can use it on 20m. I was expecting to be able to match 30m and 80m as well. But I'm not finding success in getting SWR low enough for the QMX to operate on those bands.?

Am I wrong to expect the ZM-2 to be able to present my 40m dipole as an acceptable match to the QMX (realizing that the signal going out will be partially attenuated by the tuner)?

Tnx es 73 de W2TEF

Very unlikely to ever be able to tune 80m with a 40m dipole. You could get more bands out of it if you convert the dipole to a doublet and feed it with say 300 or 450 ohm ladderline/twin lead, because that ZM-2 does balanced lines. You'll still have probs with 80m though....

On Thu, Mar 7, 2024 at 05:40 PM, Todd W2TEF wrote:

Emtech ZM-2

Just a thought on the Emtech web site... be careful with the web page; Both Nord VPN and ESET vomited out a bunch of warnings about malware on the web page.
?
--
Tisha AA4HA

On Thu, Mar 7, 2024 at 05:40 PM, Todd W2TEF wrote:

I bought an Emtech ZM-2 to use with my QMX. I'm presently trying to use it with a 40m dipole (with current balun) fed by coax. The dipole is very resonant for me on 40m. With the tuner I can use it on 20m. I was expecting to be able to match 30m and 80m as well. But I'm not finding success in getting SWR low enough for the QMX to operate on those bands.??Tnx es 73 de W2TEF

What's the height of the 40m dipole, coax RG type and length?? With that information, we can calculate mismatch loss and the complex impedance at the input end of the coax on the bands you mentioned.?

A full size 40m half-wave dipole is a quarter-wave on 80m.? Expect severe losses due to the short antenna size and coaxial mismatch loss.? It's a wasted effort even with the best low-loss tuners.

Paul, W9AC

Thanks for the responses.?

It sounds like 80m on a 40m dipole may be a quixotic pursuit. But that 30m might not be so unreasonable.?

My diople is about 33ft up, fed with about 100' of DRF-400 (with a break in the middle for a lightning arrester). It gets a perfect match on 20m (LED goes out), but only some dimming on 30m and 80m (and transmitting trips the SWR protection on the QMX). I think I got a reasonable match on 60m.

So in theory 30m should work? Are there common mistakes using or building a tuner to which I may have fallen prey? Finding success with 20m made me think I got it right, which leaves me confused why I can't use the same technique to get a match on 30m.

Todd,?
__Every tuner has a range of impedances it can match (which usually varies by band).? Your 40M dipole will be about 40-70 ohms with minimal reactance ( C or L) at resonance on 40M.? A peak at 20M finds the antenna is a center fed full wave and could approach 2500 ohms, quite a mis-match for 50 ohm coax. You are feeding it with coax that would be about 2 wavelengths long when the velocity factor of the coax is accounted for (VF is the dielectric in the coax slowing the speed of the signal's travel).? !/4 wave of coax after the feedpoint and the extreme high impedance is now just the opposite, very low.? So inside that coax length, at some point, the 'resistance' is about 50 ohms with lots of reactance.
__If you want to stick with the coax, try adding a 10 ft coax jumper in series with the line and test 30M & 20M again.? You may find the ZM2 tuner can now match the complex R +/- J impedance it 'looks' into (or not).??
__All of this gets back to the idea of feeding the antenna with open wire 300-450 ohm twinlead as Larry mentioned.? This has been the time tested way to extend what ever wire you can hang up to cover multiple bands.? I agree with Larry on 80M, you may be able to get a match, but QSO may prove difficult due to losses in the coax and tuner.
? K1TWH? < Tom

Hello,

I think that your coax might be the wrong length. Thus you end up with a very high/low impedance at the Z match, remember that if the coax impedance is not matched to the antenna impedance (which is probably the case here), impedance will vary along the coax . For instance 10m long would give you a very high impedance at the ATU end. I own the GQRP Z-match kit, don't ask me how I know this :-o
I think that you would be better of with a bifilar line and no balun (doublet antenna), depending on frequency and the coax length, the ARU + coax + balun + untuned antenna can be very inefficient. I have a 20ish doublet fed by 7m of bifilar line? (resonnant a bit below 7MHz) and it works well on all bands from 80m to 20m.

73,

Yannig - F4IUJ

I use a remote tuner at the antenna, an Icom AT140 marine tuner. It tunes my 40 meter dipole on 60, 80 and 160. Probably absorbs most of the tx power on 160.
Gary
W9TD

Thanks again for responses.?

I have 10-15ft extra feedline lying on the floor where it enters the house. It's tempting to lop that off and see if a different point in the feedline matches better (similar to adding a 10' extension?) on 30m.?

From comments above, 80m isn't going to be worth trying on the current antenna. However, spring is around the corner, and my porch within easy reach of some tall trees (where the dipole is hung) is just crying out for the use of an EFHW or a tuned random wire or experiments with doublets without worrying about fixed feedlines passing into the house.?

On Fri, Mar 8, 2024 at 04:40 AM, Todd W2TEF wrote:

"My diople is about 33ft up, fed with about 100' of DRF-400 (with a break in the middle for a lightning arrester). It gets a perfect match on 20m (LED goes out), but only some dimming on 30m and 80m (and transmitting trips the SWR protection on the QMX). I think I got a reasonable match on 60m."

I used your assumptions above to first find the complex antenna impedance at the dipole's center feed point over average soil.? 4Nec2 results:

3.55 MHz:? ? 12-j1203
10.1 MHz:? ? 347+j882
14.05 MHz:? ?6384+j436

Next, I took those values and entered them into TLD Software to computer SWR and power loss.? TLD's menu of transmission line types doesn't include the Davis RF brand of coax so I substituted Times LMR400.? For all charts below, 100W is the generated output power.? The orange-colored oval shows how much power is delivered to and radiated by the antenna.?

Moral of the story:? A 40m half-have dipole fed with 100 ft of coax, even low-loss LMR400, is a terrible multiband antenna.? However, apart from 80m, feed it with 100 ft of low-loss, balanced open feeders and magic occurs when tuned at the input end of the open transmission line.? To that point, now look at the last image below and note the?green colored oval showing the amount of power delivered to the antenna.?

Paul, W9AC

80M 100W:





30M 100W:



20M 100W:




20M 100W with 600-Ohm Open Line:

开云体育

Todd,